**1. Introduction**

26 Immunodeficiency

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[59] Seidel MG, Fritsch G, Lion T, Jurgens B, Heitger A, Bacchetta R, et al. Selective engraftment of donor CD4+25high FOXP3-positive T cells in IPEX syndrome after nonmyeloablative hematopoietic stem cell transplantation. Blood. 2009;113(22):5689-91.

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[61] Crooks BN, Taylor CE, Turner AJ, Osman HK, Abinun M, Flood TJ, et al. Respiratory viral infections in primary immune deficiencies: significance and relevance to clinical outcome in a single BMT unit. Bone marrow transplantation. 2000;26(10):1097-102. Epub

[62] Corbacioglu S, Cesaro S, Faraci M, Valteau-Couanet D, Gruhn B, Rovelli A, et al. Defibrotide for prophylaxis of hepatic veno-occlusive disease in paediatric haemopoietic stem-cell transplantation: an open-label, phase 3, randomised controlled

[63] Harkensee C, Vasdev N, Gennery AR, Willetts IE, Taylor C. Prevention and management of BK-virus associated haemorrhagic cystitis in children following haematopoietic stem cell transplantation--a systematic review and evidence-based guidance for clinical management. British journal of haematology. 2008;142(5):717-31.

[64] Muller SM, Kohn T, Schulz AS, Debatin KM, Friedrich W. Similar pattern of thymicdependent T-cell reconstitution in infants with severe combined immunodeficiency after human leukocyte antigen (HLA)-identical and HLA-nonidentical stem cell

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[66] Slatter MA, Bhattacharya A, Flood TJ, Spickett GP, Cant AJ, Abinun M, et al. Polysaccharide antibody responses are impaired post bone marrow transplantation for severe combined immunodeficiency, but not other primary immunodeficiencies. Bone

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The Human Immunodeficiency Virus-1 (HIV-1), the causative agent of the acquired immunodeficiency syndrome (AIDS), was described for the first time in 1983 [1, 2]. In the meantime, various classes of anti-retroviral drugs have been developed and combination therapy has improved the quality of life for millions of people affected. At the end of 2010 more than 34 million people were living with HIV infection worldwide [3]. Despite the increased access to antiretroviral therapy, an extensive treatment gap persists between the low-/middle-income countries and well-developed ones. This resulted in 1.8 million HIV related deaths and 2.6 million newly infected persons in 2009 [3]. Even for those who have access to treatment, there is no cure, as current therapy regimens cannot eradicate the virus. Therefore, the control and ultimate eradication of this pathogen remains one of the most important challenges in today's biomedical research.

HIV belongs to the family of *Retroviridae* and the genus *Lentivirus* (*lenti*, Latin for "slow"), which is characterized by a long incubation period causing long-lasting illnesses [4]. An HIV particle has a spherical shape and a size of about 100 nm. It consists of an outer coat, called the viral envelope, and an inner capsid enclosing two copies of positive single stranded ribonucleic acid (RNA). The RNA genome is 9.5 kB large and is composed of nine genes encoding structural (Gag, Pol and Env), regulatory (Tat and Rev) and accessory (Nef, Vif, Vpr and Vpu) proteins [5]. HIV-1 mainly infects CD4+ cells such as CD4+ T cells, macrophages and dendritic cells (DCs). Infection is initiated by the binding of the viral envelope glycoprotein 120 (gp120) to the CD4 receptor of the host cell, resulting in a conformational change that allows gp120 to interact with one of the co-receptors, CCR5 or CXCR4.

Binding of gp120 to the co-receptor induces further conformational changes that lead to the exposure of the fusion domain on glycoprotein 41 (gp41). Fusion of this domain with the

© 2012 De Haes et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

lipid cell membrane allows entry of the viral core into the host cell cytoplasm. This is followed by reverse transcription of the single-stranded RNA into double-stranded DNA, which becomes integrated into the host genome [6]. After DNA integration, HIV remains present as a latent DNA provirus which becomes active upon cell activation [7]. Transcription of the viral proteins eventually leads to the formation of mature and infectious virions [8].

"Wrapped Up" Vaccines in the Context of HIV-1 Immunotherapy 29

1 % of infected individuals have viral loads below 50 copies per ml for at least 1-2 years while untreated (elite controllers or HIV controllers) [15]. Some individuals remain

The first effective antiretroviral drugs (all nucleoside analogues) became available at the end of the nineties. They were used in single and later in dual combinations, but could suppress the viral load (VL) only temporarily. This was due to the appearance of drug resistance [17]. Triple drug combinations called "highly active antiretroviral therapy" (HAART), are able to suppress VL in a more sustained way and hence can prevent the emergence of drug resistance. For a number of years viral suppression was only possible at the cost of a high medication burden and many side effects. During the last decade, however, HAART has become less complicated and better tolerated, which has converted HIV-1 infection into a

It should be kept in mind, however, that HAART is not a treatment devoid of shortcomings. Firstly, a life-long commitment to the therapy remains mandatory to keep the virus under control and delay disease progression. Secondly, the treatment may cause toxic drug-related adverse effects such as cardiovascular complications [19], renal and hepatic diseases [20], lipodystrophy and diabetes mellitus, collectively called "metabolic syndrome" [21]. Thirdly, even though HAART restores the number of circulating CD4+ T cells to near normal levels, responses against HIV itself remain deficient [11, 22]. Finally, this costly treatment is not available for all infected persons, especially not in low- and middle-income countries in Africa, Asia and Latin America, where the numbers of patients are the highest and still

Therefore, there is a clear need for cheaper and more widely available therapies that can suppress and/or eliminate the viral reservoir even if the treatment is stopped or interrupted. Improving HIV-specific cell-mediated immunity by therapeutic vaccination is a generally

During the last decade immunotherapeutic vaccination strategies have been sought after to boost the immune system in order to control virus replication and to eliminate infected cells. These vaccines are largely based on ex vivo loading of dendritic cells with antigens and immune-stimulating molecules. This personalized process is time-consuming, labor intensive, and requires strict quality control. It should be stressed that high costs of the procedure together with the need to use sophisticated equipment, restrains its application in

Recently, particulate antigen vehicles have been introduced in the field of vaccine design with the purpose to improve antigen delivery and to induce antigen specific immune responses. A variety of nano- and micro-carriers has been developed to deliver protein, peptides and/or nucleic acids to cells of the immune system. The intracellular fate of these particles depends on their physicochemical characteristics such as size, stability and charge. These in turn determine the efficiency with which the specific cargo is delivered to antigen-

presenting cells (APC) and the extent of antigen-specific immune responses induced.

uninfected, despite being highly exposed to HIV-1 (exposed seronegatives) [16].

chronic but treatable disease [18].

accepted approach to tackle the problem.

less developed countries.

increasing [3].

HIV-1 can be transmitted "horizontally" through hetero- or homosexual contact or bloodblood contact (e.g. blood transfusion or intravenous drug use) as well as "vertically" from mother-to-child [9]. The transmission of HIV strongly depends on the concentration of virus in the body fluids (genital secretions, plasma or breast milk), viral "fitness" properties and the host susceptibility at both the immunological and the cellular level [10].

Clinically, an HIV-1 infection course can be divided in three stages: the primary or acute infection phase, the chronic phase (first asymptomatic and later non-AIDS defining symptoms) and the terminal AIDS defining illness. The first days after infection, the virus spreads from the portal of entry via regional lymph nodes throughout the body. It readily infects CD4+ T cells, producing new virions, which results in a high plasma viral load (VL). The virus finds its way to all lymphoid organs, but with a particularly massive viral production by the gastro-intestinal associated lymphoid tissue (GALT) is often observed. Remarkably, only a proportion of the newly infected patients experiences a clinical "acute phase syndrome", characterized by mononucleosis- or flu-like symptoms, including fever, fatigue, sore throat, skin rash, enlarged lymph nodes, diarrhea, nausea and general malaise. In the first three to six weeks a rapid decline of CD4+ T cells is observed in the peripheral blood and even more pronounced in the GALT, which nonetheless remains an important HIV reservoir [11]. The appearance of HIV-1 specific cellular immune responses and the subsequent production of HIV-1 specific antibodies results in a sharp drop of viral load reaching a steady state viraemia, called the viral setpoint (usually within six months after infection). A dynamic equilibrium is then established between viral replication (fitness) and viral suppression by the immune system. Nevertheless, because of the limited regeneration capacity of the immune system (including thymic atrophy in adults) the number of CD4+ T cells will continue to gradually decrease during the chronic phase. This stage can last up to ten years and is characterized by lack of clinical symptoms of illness or relatively mild symptoms that often do not raise suspicion of HIV infection. Eventually the immune system becomes exhausted due to chronic immune activation and T-cell depletion as a result of direct cytopathic effects of infected cells, but even more by induction of apoptosis of uninfected bystander cell (CD4+ and CD8+ T cells) and degeneration of lymphoid organs. Opportunistic diseases, including serious infections or malignant tumors that are no longer controlled due to a loss of immune surveillance are the cause of AIDS-related deaths [10, 12, 13].

The majority (> 90 %) of infected individuals progresses to AIDS within about ten years after primary infection (normal progressors). Some individuals (around 5 %) remain asymptomatic for more than ten years with stable numbers of CD4+ T cells and low to intermediate viral loads (long term survivors and long term non progressors) [14]. Less than 1 % of infected individuals have viral loads below 50 copies per ml for at least 1-2 years while untreated (elite controllers or HIV controllers) [15]. Some individuals remain uninfected, despite being highly exposed to HIV-1 (exposed seronegatives) [16].

28 Immunodeficiency

virions [8].

lipid cell membrane allows entry of the viral core into the host cell cytoplasm. This is followed by reverse transcription of the single-stranded RNA into double-stranded DNA, which becomes integrated into the host genome [6]. After DNA integration, HIV remains present as a latent DNA provirus which becomes active upon cell activation [7]. Transcription of the viral proteins eventually leads to the formation of mature and infectious

HIV-1 can be transmitted "horizontally" through hetero- or homosexual contact or bloodblood contact (e.g. blood transfusion or intravenous drug use) as well as "vertically" from mother-to-child [9]. The transmission of HIV strongly depends on the concentration of virus in the body fluids (genital secretions, plasma or breast milk), viral "fitness" properties and

Clinically, an HIV-1 infection course can be divided in three stages: the primary or acute infection phase, the chronic phase (first asymptomatic and later non-AIDS defining symptoms) and the terminal AIDS defining illness. The first days after infection, the virus spreads from the portal of entry via regional lymph nodes throughout the body. It readily infects CD4+ T cells, producing new virions, which results in a high plasma viral load (VL). The virus finds its way to all lymphoid organs, but with a particularly massive viral production by the gastro-intestinal associated lymphoid tissue (GALT) is often observed. Remarkably, only a proportion of the newly infected patients experiences a clinical "acute phase syndrome", characterized by mononucleosis- or flu-like symptoms, including fever, fatigue, sore throat, skin rash, enlarged lymph nodes, diarrhea, nausea and general malaise. In the first three to six weeks a rapid decline of CD4+ T cells is observed in the peripheral blood and even more pronounced in the GALT, which nonetheless remains an important HIV reservoir [11]. The appearance of HIV-1 specific cellular immune responses and the subsequent production of HIV-1 specific antibodies results in a sharp drop of viral load reaching a steady state viraemia, called the viral setpoint (usually within six months after infection). A dynamic equilibrium is then established between viral replication (fitness) and viral suppression by the immune system. Nevertheless, because of the limited regeneration capacity of the immune system (including thymic atrophy in adults) the number of CD4+ T cells will continue to gradually decrease during the chronic phase. This stage can last up to ten years and is characterized by lack of clinical symptoms of illness or relatively mild symptoms that often do not raise suspicion of HIV infection. Eventually the immune system becomes exhausted due to chronic immune activation and T-cell depletion as a result of direct cytopathic effects of infected cells, but even more by induction of apoptosis of uninfected bystander cell (CD4+ and CD8+ T cells) and degeneration of lymphoid organs. Opportunistic diseases, including serious infections or malignant tumors that are no longer controlled due to

the host susceptibility at both the immunological and the cellular level [10].

a loss of immune surveillance are the cause of AIDS-related deaths [10, 12, 13].

The majority (> 90 %) of infected individuals progresses to AIDS within about ten years after primary infection (normal progressors). Some individuals (around 5 %) remain asymptomatic for more than ten years with stable numbers of CD4+ T cells and low to intermediate viral loads (long term survivors and long term non progressors) [14]. Less than The first effective antiretroviral drugs (all nucleoside analogues) became available at the end of the nineties. They were used in single and later in dual combinations, but could suppress the viral load (VL) only temporarily. This was due to the appearance of drug resistance [17]. Triple drug combinations called "highly active antiretroviral therapy" (HAART), are able to suppress VL in a more sustained way and hence can prevent the emergence of drug resistance. For a number of years viral suppression was only possible at the cost of a high medication burden and many side effects. During the last decade, however, HAART has become less complicated and better tolerated, which has converted HIV-1 infection into a chronic but treatable disease [18].

It should be kept in mind, however, that HAART is not a treatment devoid of shortcomings. Firstly, a life-long commitment to the therapy remains mandatory to keep the virus under control and delay disease progression. Secondly, the treatment may cause toxic drug-related adverse effects such as cardiovascular complications [19], renal and hepatic diseases [20], lipodystrophy and diabetes mellitus, collectively called "metabolic syndrome" [21]. Thirdly, even though HAART restores the number of circulating CD4+ T cells to near normal levels, responses against HIV itself remain deficient [11, 22]. Finally, this costly treatment is not available for all infected persons, especially not in low- and middle-income countries in Africa, Asia and Latin America, where the numbers of patients are the highest and still increasing [3].

Therefore, there is a clear need for cheaper and more widely available therapies that can suppress and/or eliminate the viral reservoir even if the treatment is stopped or interrupted. Improving HIV-specific cell-mediated immunity by therapeutic vaccination is a generally accepted approach to tackle the problem.

During the last decade immunotherapeutic vaccination strategies have been sought after to boost the immune system in order to control virus replication and to eliminate infected cells. These vaccines are largely based on ex vivo loading of dendritic cells with antigens and immune-stimulating molecules. This personalized process is time-consuming, labor intensive, and requires strict quality control. It should be stressed that high costs of the procedure together with the need to use sophisticated equipment, restrains its application in less developed countries.

Recently, particulate antigen vehicles have been introduced in the field of vaccine design with the purpose to improve antigen delivery and to induce antigen specific immune responses. A variety of nano- and micro-carriers has been developed to deliver protein, peptides and/or nucleic acids to cells of the immune system. The intracellular fate of these particles depends on their physicochemical characteristics such as size, stability and charge. These in turn determine the efficiency with which the specific cargo is delivered to antigenpresenting cells (APC) and the extent of antigen-specific immune responses induced.

#### **2. HIV and the immune system**

#### **2.1. Immune activation**

Chronic HIV-related immune activation is characterized by the inappropriate production of pro-inflammatory cytokines and overexpression of cellular activation and exhaustion markers. Most of these inflammatory responses induced by HIV are not directed toward HIV. They rather enhance susceptibility of target cells to HIV infection and enhance virus replication in already infected cells, which accelerates disease progression. This chronic, non-specific T cell activation leads to T cell exhaustion and apoptosis of CD4+ and CD8+ T cells [23]. Increased expressions of HLA-DR and CD38 molecules on CD8+ T cells correlate with a higher level of immune activation and constitute markers for bad prognosis, which are partly independent from actual CD4 T count and VL [24].

"Wrapped Up" Vaccines in the Context of HIV-1 Immunotherapy 31

macrophages and B cells) that digest the proteins into small peptides and present them on MHC II molecules to CD4+ T helper (Th) cells. Specifically activated Th2 cells that produce B cell stimulating cytokines (including IL-4, IL-5, IL-6, IL-10, TGF−β) will activate naive B cells. The latter are recognized by specific epitopes or intact virus through their surface IgM and promote B cell differentiation into plasma cells producing large amounts of IgG, IgA, IgE antibodies and memory B cells. During HIV-1 infection antibodies against gp120, gp41, the nucleocapsid (p24) and the matrix (p17) arise few weeks to several months after infection.

**Figure 1.** Overview of the adaptive immune responses after virus recognition by antigen presenting cells. Virus antigens are presented by dendritic cells and B cells to T cells. Infected cells present peptides together with MHC I molecules on the plasma membrane. The peptide-MHC I complex is recognized by precursor cytotoxic CD8+ T lymphocytes (CTLs). Th1 cells, induced by antigen presenting cells, produce IL-2, IFN-γ, and TNF-α. This results in activation and differentiation of the precursor CTLs into memory or effector CTLs. Effector CTLs can directly kill infected cells by the production of perforines and granzymes. Activated Th2 cells, also induced by antigen presenting cells, produce B cell

stimulating cytokines (including IL-4, IL-5, IL-6, IL-10, TGF−β) that activate naive B cells. This facilitates/induces B cell differentiation into memory B cells and plasma cells that produce large amounts of IgG, IgA, IgE antibodies that prevent further virus infection. Ab: antibody, Ag: antigen, APC: Antigen Presenting Cell, DC: Dendritic Cell, IL: Interleukin, TCR: T Cell Receptor, Th: CD4+ T

helper cell.

This process is commonly referred to as seroconversion.

It remains unclear whether there is a single key mechanism behind this HIV-associated immune activation. A so-called "leaky gut syndrome" hypothesis proposes that massive loss of CD4+ T cells in the GALT may affect the protective barrier of the intestinal mucosa, allowing bacterial toxins such as lipopolysaccharide (LPS) to enter the bloodstream [25]. This "microbial translocation" could in consequence induce a pathological over-activation of both the innate and adaptive immune system. Another hypothesis puts more emphasis on intrinsic regulation of type I interferon (IFN) [26]. It has been shown indeed that patterns of type I IFNs produced by plasmacytoid DCs (pDCs) are different in non-pathogenic SIV infections of natural hosts (like sooty mangabeys, African green monkeys and mandrills) and pathogenic SIV infections of rhesus macaques. High and robust type I IFN responses are observed in natural hosts during acute infection. Expression of type I IFNs is, however, down-regulated during the chronic infection phase. By contrast, the type I IFNs are persistently produced in SIV infected rhesus macaques [27]. Sooty mangabeys, the natural hosts of SIV, show no immune activation and rarely progress to AIDS, despite high levels of virus replication and severe CD4+ T cell depletion in the GALT. In contrast, rhesus macaques, infected with the same or closely related SIV, progress to AIDS [28].

Another enigma remains the role of CD25 and forkhead box (FOX) P3 expressing regulatory CD4+ T (Treg) cells. On the one hand, they may suppress chronic immune activation. On the other hand, they could undermine the effective T cell responses [29]. It has been shown that the number of Treg cells increases in the GALT, but not in the peripheral blood, during HIV infection in untreated individuals [30]. Whether this accumulation of Treg cells delays disease progression by inhibition of immune activation or increases the susceptibility of the gastrointestinal tract to opportunistic infections remains a matter of debate [29].

#### **2.2. HIV-specific humoral immune response**

The humoral immune response is mediated by antibody producing B cells (**figure 1**). In general, by preventing infections of the host cells, virus-specific antibodies play an important role in the control of many viral infections [31]. This arm of the adaptive immune system is activated after uptake of viral proteins by antigen presenting cells (DCs, macrophages and B cells) that digest the proteins into small peptides and present them on MHC II molecules to CD4+ T helper (Th) cells. Specifically activated Th2 cells that produce B cell stimulating cytokines (including IL-4, IL-5, IL-6, IL-10, TGF−β) will activate naive B cells. The latter are recognized by specific epitopes or intact virus through their surface IgM and promote B cell differentiation into plasma cells producing large amounts of IgG, IgA, IgE antibodies and memory B cells. During HIV-1 infection antibodies against gp120, gp41, the nucleocapsid (p24) and the matrix (p17) arise few weeks to several months after infection. This process is commonly referred to as seroconversion.

30 Immunodeficiency

**2. HIV and the immune system** 

are partly independent from actual CD4 T count and VL [24].

Chronic HIV-related immune activation is characterized by the inappropriate production of pro-inflammatory cytokines and overexpression of cellular activation and exhaustion markers. Most of these inflammatory responses induced by HIV are not directed toward HIV. They rather enhance susceptibility of target cells to HIV infection and enhance virus replication in already infected cells, which accelerates disease progression. This chronic, non-specific T cell activation leads to T cell exhaustion and apoptosis of CD4+ and CD8+ T cells [23]. Increased expressions of HLA-DR and CD38 molecules on CD8+ T cells correlate with a higher level of immune activation and constitute markers for bad prognosis, which

It remains unclear whether there is a single key mechanism behind this HIV-associated immune activation. A so-called "leaky gut syndrome" hypothesis proposes that massive loss of CD4+ T cells in the GALT may affect the protective barrier of the intestinal mucosa, allowing bacterial toxins such as lipopolysaccharide (LPS) to enter the bloodstream [25]. This "microbial translocation" could in consequence induce a pathological over-activation of both the innate and adaptive immune system. Another hypothesis puts more emphasis on intrinsic regulation of type I interferon (IFN) [26]. It has been shown indeed that patterns of type I IFNs produced by plasmacytoid DCs (pDCs) are different in non-pathogenic SIV infections of natural hosts (like sooty mangabeys, African green monkeys and mandrills) and pathogenic SIV infections of rhesus macaques. High and robust type I IFN responses are observed in natural hosts during acute infection. Expression of type I IFNs is, however, down-regulated during the chronic infection phase. By contrast, the type I IFNs are persistently produced in SIV infected rhesus macaques [27]. Sooty mangabeys, the natural hosts of SIV, show no immune activation and rarely progress to AIDS, despite high levels of virus replication and severe CD4+ T cell depletion in the GALT. In contrast, rhesus

macaques, infected with the same or closely related SIV, progress to AIDS [28].

gastrointestinal tract to opportunistic infections remains a matter of debate [29].

**2.2. HIV-specific humoral immune response** 

Another enigma remains the role of CD25 and forkhead box (FOX) P3 expressing regulatory CD4+ T (Treg) cells. On the one hand, they may suppress chronic immune activation. On the other hand, they could undermine the effective T cell responses [29]. It has been shown that the number of Treg cells increases in the GALT, but not in the peripheral blood, during HIV infection in untreated individuals [30]. Whether this accumulation of Treg cells delays disease progression by inhibition of immune activation or increases the susceptibility of the

The humoral immune response is mediated by antibody producing B cells (**figure 1**). In general, by preventing infections of the host cells, virus-specific antibodies play an important role in the control of many viral infections [31]. This arm of the adaptive immune system is activated after uptake of viral proteins by antigen presenting cells (DCs,

**2.1. Immune activation** 

**Figure 1.** Overview of the adaptive immune responses after virus recognition by antigen presenting cells. Virus antigens are presented by dendritic cells and B cells to T cells. Infected cells present peptides together with MHC I molecules on the plasma membrane. The peptide-MHC I complex is recognized by precursor cytotoxic CD8+ T lymphocytes (CTLs). Th1 cells, induced by antigen presenting cells, produce IL-2, IFN-γ, and TNF-α. This results in activation and differentiation of the precursor CTLs into memory or effector CTLs. Effector CTLs can directly kill infected cells by the production of perforines and granzymes. Activated Th2 cells, also induced by antigen presenting cells, produce B cell stimulating cytokines (including IL-4, IL-5, IL-6, IL-10, TGF−β) that activate naive B cells. This facilitates/induces B cell differentiation into memory B cells and plasma cells that produce large amounts of IgG, IgA, IgE antibodies that prevent further virus infection. Ab: antibody, Ag: antigen, APC: Antigen Presenting Cell, DC: Dendritic Cell, IL: Interleukin, TCR: T Cell Receptor, Th: CD4+ T helper cell.

The virus neutralization is characterized by the interaction of specific antibodies with the viral envelope spikes. This interferes with virus attachment or viral entry in target cells and results in the inhibition of infection. Only a minority of anti-HIV Env antibodies, at any time, exerts immune pressure by autologous neutralization. However, the virus easily mutates and readily escapes from these potentially protective immune responses [32]. During the chronic course of infection only 20% of the infected individuals will generate broadly neutralizing antibodies (bNAbs) having the ability to neutralize heterologous viruses [33]. In addition to classical neutralization, antibodies can attach to HIV infected cells and kill them via antibody dependent cellular cytotoxicity (ADCC) mediated through their Fc moiety and natural killers cells (NK) [34, 35].

"Wrapped Up" Vaccines in the Context of HIV-1 Immunotherapy 33

The first T cell responses during HIV infection arise when the viraemia peak is approached and reach maximum 1-2 weeks later. In non-controllers, the virus evades the CD8+ mediated T cell response by introducing mutations in CTL epitopes [39], by Nef-mediated down-regulation of MHC I and by influencing cytokine production and T-cell signaling [40]. Since an optimal CD8+ T cell response, similar to the B cell response, depends on help of CD4+ T lymphocytes, the deterioration of CD8 mediated viral control is also related to the

There are many indications that HIV-specific CD8+ T cell responses are responsible for at least partial VL control. In the macaque model, depletion of CD8+ T cells during SIVinfection resulted in an increased viral load [43, 44]. In HIV-infected human subjects, who initially control the virus, escape mutations in specific CD8+ T cell epitopes were responsible

To date no definite biological markers have been unambiguously shown to correlate with patients' ability to control HIV infection by suppressing virus production and eliminating infected cells. Defining these factors would be crucial for the development of preventive and therapeutic vaccination strategies. For that reason, research groups focusing on preventive vaccines carefully study individuals that can avoid infection (exposed seronegatives) or

Individuals that have the homozygote deletion Δ32 in co-receptor CCR5 are largely resistant to HIV-1 infection [47]. It has been recently reported that an HIV patient, who received *CCR5* Δ32/Δ32 stem cells for transplantation, remained without viral rebound for several

Certain intracellular molecules, expressed by the host, can at least partly protect against cellular infection or virus release. The most important factors identified so far are APOBEC3G, APOBEC3F, TRIM5α and tetherin. APOBEC3G is a cytosine deaminase that incorporates adenosine instead of guanosine during synthesis of the viral DNA, which results in defective proviral DNA [50]. In addition APOBEC3G promotes natural killer cellmediated lysis [51]. Individuals expressing large amounts of APOBEC3G have lower viral loads during acute infection phase [52]. TRIM5a binds to the viral capsid, blocking replication early in the viral life cycle [53]. Tetherin interferes with the virion release by

Certain polymorphisms in the human leukocyte antigen (HLA type) and T or NK cell receptor can affect the cellular HIV-specific immune responses. It has been shown that human leukocyte antigens B\*27, B\*57 and B\*58 are associated with better control of HIV-1 and slower disease progression [55-57]. Interestingly HLA B\*57 also plays a role in the innate protective immune responses, acting as a natural ligand for inhibitory killer

attaching the mature virions to each other and to the host membrane [54].

weakening of CD4+ T cell function [41, 42].

for the loss of control and increase in VL [45, 46].

partly control the virus load without the need of HAART.

**3.1. Natural resistance and genetic factors of the host** 

**3. Correlates of protection** 

years [48, 49].

#### **2.3. HIV-specific cellular immune response**

The cellular immune response is the other arm of the adaptive immune system (**figure 1**) and it is crucial to combat viral infections. CD8+ cytotoxic T lymphocytes (CTLs), which eliminate infected cells, play a key role in this process. The initial step involves processing of intracellular antigens by the proteasome. The resulting peptides are then presented together with MHC I molecules on the membrane of infected somatic cells. The peptide-MHC I complex is recognized by precursor cytotoxic CD8+ T lymphocytes (CTLs). Also in that case a CD4+ T cell help, induced by antigen presenting cells, is crucial. In this case so-called Th1 cells, producing IL-2, IFN-γ, and TNF-α, activate and differentiate the CTLs into memory or effector CTLs. Effector CTLs can directly kill infected cells by the production of perforines and granzymes (**figure 2**) [36]. Alternatively, CTLs can induce apoptosis of the infected cells after interaction of Fas ligand on CTLs with Fas receptor on infected T cells [37]. CD8+ T cells also display a non-cytotoxic antiviral activity involving several cytokines, chemokines and a yet unidentified soluble CD8+ cell antiviral factor (CAF) [38].

**Figure 2.** HIV-induced T cell responses. HIV specific CD8+ effector cells produce chemokines and cytokines in order to eliminate infected cells. CD4+ T helper cells help to stimulate both dendritic cells and CD8+ T-cells to maintain a CD8+ T-cell memory response. HIV interferes with this supportive function of CD4+ T-cells.

The first T cell responses during HIV infection arise when the viraemia peak is approached and reach maximum 1-2 weeks later. In non-controllers, the virus evades the CD8+ mediated T cell response by introducing mutations in CTL epitopes [39], by Nef-mediated down-regulation of MHC I and by influencing cytokine production and T-cell signaling [40]. Since an optimal CD8+ T cell response, similar to the B cell response, depends on help of CD4+ T lymphocytes, the deterioration of CD8 mediated viral control is also related to the weakening of CD4+ T cell function [41, 42].

There are many indications that HIV-specific CD8+ T cell responses are responsible for at least partial VL control. In the macaque model, depletion of CD8+ T cells during SIVinfection resulted in an increased viral load [43, 44]. In HIV-infected human subjects, who initially control the virus, escape mutations in specific CD8+ T cell epitopes were responsible for the loss of control and increase in VL [45, 46].
