**4. Immunology of HIV**

#### **4.1. Entry through the mucosal barrier**

The principal route of HIV entry into the human body is through the mucosal epithelial surfaces (mainly genital or rectal). The virus prefers to infect immune cells expressing CD4+ receptors on their surface—CD4+ helper T cells, macrophages, and dendritic cells—all present in activated state in the mucosal lymphoid tissues.

HIV crosses the intact epithelial layers via interaction with dendritic cells and/or CD4+ helper T cells. The CD4+ receptor of these cells recognizes the viral envelope glycoprotein 120 (gp120) and causes its conformational change. This makes possible the binding with one of the auxiliary co-receptors: CCR5 or CXCR4. The second binding allows viral glycoprotein 41 (gp41) to integrate within the cell membrane and viral genome to access the host cytoplasm.

After entry into the cell, viral RNA—under the action of reverse transcriptase and integrase turns into double-strand DNA and integrates into the host DNA. The integrated copy of HIV nucleic acid (named provirus) can stay in latent form for years and can become active at any time to complete the viral life cycle and produce new virions.

The principal target for HIV is the activated CD4+ T-lymphocyte expressing CCR5 or CXCR4 co-receptor. The high number of CD4+ T cells (especially memory CD4+CCR5+ T-lymphocytes) in the mucosa is ideal for HIV replication [37]. In contrast, mucosal dendritic cells do not express CD4+ or CCR5 receptors [38] but they do express other receptors that successfully recognize HIV envelope proteins—dendritic cell-specific intercellular adhesion molecule grabbing nonintegrins (DC-SIGN) and langerin [39].

#### **4.2. HIV expansion**

Early after the entry of HIV, the primary infected T-lymphocytes attract and activate other immune cells to expand the initial place of infection. The infected local site starts to grow until the detachment of the infected cells. Thus, the virus disseminates from the mucosal surfaces to the regional lymph nodes (usually in 10 days after the infection) and then throughout the body. The virus can be detected in all lymphoid organs but the highest number of viral particles concentrates in the mucosal lymphoid tissues (such as gastro-intestinal associated lymphoid tissue). Dendritic cells in lymph nodes and mucosal lymphoid tissues present the HIV antigens to naïve B cells and T cells, prompting them to differentiate and activate. In these first stages, the viral load is significant and CD4+ T cells count starts to decline rapidly.

In many cases, the early events of viral expansion (acute stage) are not clinically manifested. In some people, influenza-like or mononucleosis-like symptoms may occur several weeks after the exposure to infection but they are not specific and often misdiagnosed.

#### **4.3. T cell-mediated immune response during HIV latency**

Several months after the initial infection, a precise balance between the virus and the immune response (both cell-mediated and humoral) is established. Host immune system (especially CD8+ lymphocytes) succeeds to control viral replication at some degree but the elimination of target CD4+ population continues. This represents the chronic (latent) phase in HIV infection, clinically manifested with decreased viral load and appearance of specific anti-HIV antibodies in the blood of infected individuals.

Although the major characteristic of HIV infection is the depletion of CD4+ T cells, other lymphocyte types are also affected. HIV notably influences naïve T cells (both CD4 T naïve cells and CD8 T naïve cells) during the asymptomatic phase of the infection [40]. Any T cellmediated immune response requires new naïve T cells and their depletion results in progressively compromised immunity. The reduction in naïve T cell counts is probably due to decreased thymic output and increased T cell turnover in HIV-positive patients [41]. Furthermore, during HIV asymptomatic phase, the overall CD8+ count remains the same as before infection but significant alteration in the ratio memory/naïve CD8+ cells occurs, the normal predominance of naïve CD8+ is replaced by the domination of memory cells (in many patients, the compartment of CD8+ memory cells represents more than 80% of all CD8+ circulating cells) [40].

The cellular immune response mediated by CD8+ cytotoxic T-lymphocytes (CTLs) plays a crucial role in HIV infection [42]. The precursor CTLs recognize viral antigens expressed on the surface of presenting cells. They become active with the help of IL-2, IFN-gamma, and TNF-alpha (produced from CD4+ T cells) and can directly eliminate infected CD4+ lympho‐ cytes via secretion of perforin (a cytolytic protein) and granzymes (serine proteases). The increased ability to secrete perforin is specific for CD8+ T cells from small group HIV-positive individuals who maintain HIV replication in undetectable levels (elite controllers) [43] and who do not progress immunologically towards AIDS in the absence of antiretroviral treatment.

Simultaneously with the host response, the virus starts a mutation process to escape CD8+ recognition by changing epitopes and to correct the present viral variant. At this moment, a balance between host elimination and replication of new HIV variants is established. However, CD8+ immune response, as well as humoral response, strongly needs CD4+ T helpers and gradually with CD4+ depletion, the CD8+ control fails to maintain the viremia.

The loss of effective antiviral response leads to a constant stimulation and repeated activation of HIV specific CD8+ T cells. This chronic activation soon exhausts the pool of CD8+ lympho‐ cytes together with CD4+ T cell one.

The elevated activation is characteristic not only for CD8+ and CD4+ T cells, but also for B cells, NK cells, and monocytes. In HIV-seropositive patients, even in early stages of infection, high levels of proinflammatory cytokines (TNF-alpha, IL-6 and IL-1β) and chemokines (MIP-1al‐ pha, MIP-1beta and RANTES) are detected [37].

The HIV life cycle is closely related to the extent of immune system activation. Activation of the immune cells enhances the entry of HIV into the cytoplasm because the auxiliary coreceptors are upregulated under immune stimulation [44]. It also increases the level of provirus transcription together with the normal cellular transcription [45]. The exhausted CD8+ T cells diminish secretion of immune-stimulatory cytokines, slow down proliferation and suspend killing of infected cells [46]. HIV infection stimulates upregulation of CD8+ inhibitory receptors (Programmed Death-1 (PD-1), T cell immunoglobulin, and mucindomain-containing molecule-3 (Tim-3) [47,48]) that downregulate CD8+ immune response and maintain T cell tolerance.

#### **4.4. Humoral immune response against HIV**

The humoral immune response against HIV is mediated by specific anti-HIV antibodies produced from B-lymphocytes several weeks to several months after the initial infection. HIV cannot replicate into B-lymphocytes but influences their activation and apoptosis. Unmatured B-lymphocytes differentiate under the action of Th1 cytokines (IL-4, IL-5, IL-6, IL-10, TGF-beta) into plasma cells producing large amounts of binding and virus-neutralizing antibodies. Antibodies target free viral particles in the HIV-positive blood and a small quantity of them can also eliminate infected cells.

HIV specific antibodies recognize viral envelope (envelope proteins gp120 and gp41) and protect host cells from viral entry, but the virus successfully escapes the antibodies' binding via continual mutations. Memory B cells constantly produce new variants of antibodies and soon become exhausted.

#### **4.5. AIDS**

the detachment of the infected cells. Thus, the virus disseminates from the mucosal surfaces to the regional lymph nodes (usually in 10 days after the infection) and then throughout the body. The virus can be detected in all lymphoid organs but the highest number of viral particles concentrates in the mucosal lymphoid tissues (such as gastro-intestinal associated lymphoid tissue). Dendritic cells in lymph nodes and mucosal lymphoid tissues present the HIV antigens to naïve B cells and T cells, prompting them to differentiate and activate. In these first stages,

In many cases, the early events of viral expansion (acute stage) are not clinically manifested. In some people, influenza-like or mononucleosis-like symptoms may occur several weeks after

Several months after the initial infection, a precise balance between the virus and the immune response (both cell-mediated and humoral) is established. Host immune system (especially CD8+ lymphocytes) succeeds to control viral replication at some degree but the elimination of target CD4+ population continues. This represents the chronic (latent) phase in HIV infection, clinically manifested with decreased viral load and appearance of specific anti-HIV antibodies

Although the major characteristic of HIV infection is the depletion of CD4+ T cells, other lymphocyte types are also affected. HIV notably influences naïve T cells (both CD4 T naïve cells and CD8 T naïve cells) during the asymptomatic phase of the infection [40]. Any T cellmediated immune response requires new naïve T cells and their depletion results in progressively compromised immunity. The reduction in naïve T cell counts is probably due to decreased thymic output and increased T cell turnover in HIV-positive patients [41]. Furthermore, during HIV asymptomatic phase, the overall CD8+ count remains the same as before infection but significant alteration in the ratio memory/naïve CD8+ cells occurs, the normal predominance of naïve CD8+ is replaced by the domination of memory cells (in many patients, the compartment of CD8+ memory cells represents more than 80% of all CD8+

The cellular immune response mediated by CD8+ cytotoxic T-lymphocytes (CTLs) plays a crucial role in HIV infection [42]. The precursor CTLs recognize viral antigens expressed on the surface of presenting cells. They become active with the help of IL-2, IFN-gamma, and TNF-alpha (produced from CD4+ T cells) and can directly eliminate infected CD4+ lympho‐ cytes via secretion of perforin (a cytolytic protein) and granzymes (serine proteases). The increased ability to secrete perforin is specific for CD8+ T cells from small group HIV-positive individuals who maintain HIV replication in undetectable levels (elite controllers) [43] and who do not progress immunologically towards AIDS in the absence of antiretroviral treatment.

Simultaneously with the host response, the virus starts a mutation process to escape CD8+ recognition by changing epitopes and to correct the present viral variant. At this moment, a balance between host elimination and replication of new HIV variants is established. However,

the viral load is significant and CD4+ T cells count starts to decline rapidly.

the exposure to infection but they are not specific and often misdiagnosed.

**4.3. T cell-mediated immune response during HIV latency**

in the blood of infected individuals.

62 Immunopathology and Immunomodulation

circulating cells) [40].

The latent phase can last several years (usually up to ten) and terminates finally with the development of AIDS and death. In the final stage, HIV infection completely exhausts the host immune system and especially the CD4+ T cell pool. AIDS is immunologically diagnosed by a CD4+ count less than 200 per µL or/and clinically by the occurrence of opportunistic HIVrelated diseases.
