**5. Pathobiology of immunodeficiency in malnutrition**

Malnutrition is considered to be the commonest cause of immunodeficiency in the world. It adversely impacts every aspect of immune function. All these immune dysfunctions are collectively referred to as nutritional-acquired immunodeficiency syndrome (NAIDS). Understanding of malnutrition-related immunodeficiency can shed a lot of insight into immunodeficiency of HIV/AIDS.

Profound thymic atrophy with depletion of thymocytes and changes in thymic extracellular matrix are seen even in moderate malnutrition. It is however difficult to say whether these changes in thymic function are due to malnutrition *per se* or due to the severe infections frequently associated with malnutrition. Changes in thymic micro-environment like decreased thymic epithelial cells, expansion of extracellular matrix and decreased production of thymic hormone all contribute to thymic depletion. Thymocyte depletion results from increased apoptosis of CD4 and CD8 double-positive, double-negative and single-positive (immature) thymic lymphocytes. Apoptosis is driven by increased circulating levels of glucocorticoids, reduced leptin levels and deficiency of dietary protein and zinc.

Bone marrow cellularity is reduced, its stroma altered and there is limitation of extra-cellular matrix expansion. A study on bone marrow changes in children with PEM showed erythroid hypoplasia/dysplasia in the marrows of 50% children with kwashiorkor, 30% children with marasmic-kwashiorkor and 28.5% children with marasmus [114]. Suppression of cell cycle progression of haematopoietic progenitor cells with cell cycle arrest in G0/G1 phase is seen in protein malnutrition. This results in reduction in red cell and white cell lineages. In addition, bone marrow granulocytes display impaired blastic response to granulocyte-colony stimulating factor (G-CSF) and suboptimal mobilisation on lipopolysaccharide challenge. In protein-deficient mice models, bone marrow mesenchymal cells tend to differentiate into adipose cells, thereby altering the cytokine micro-environment in the bone marrow and compromising haematopoiesis. Despite this, the total number of leucocytes in peripheral blood of children with severe acute malnutrition remains normal. However, the number of dendritic cells is reduced [115]. Mice with transferrin receptor-1 deficiency are unable to absorb adequate iron. This results in impaired T-cell development and fewer mature B cells [116].

 Secondary lymphoid tissue in the spleen and lymph nodes shows similar degenerative and hypo-proliferative changes in mouse protein-deficient models. The spleen has a thickened capsule and is deficient in splenocytes and splenic mononuclear cells. Cell cycle arrest, similar to that seen in the bone marrow, is seen. Splenic white pulp is also disorganised. Changes in lymph nodes can be seen even in moderate malnutrition. Zinc and iron deficiencies exaggerate changes caused by protein-energy malnutrition. There is hypoplasia of lymph nodes, decreased number of dendritic cells, macrophages, neutrophils and fibroblasts. The ability of lymph nodes to act as an effective barrier to pathogen spread is compromised. Poor trafficking of soluble antigens through the lymphoid conduits is also seen.

It would be intuitive to assume that like all other lymphoid tissue, the gutassociated lymphoid tissue (GALT) should also be hypoplastic in malnutrition. This has not been shown in humans conclusively.

#### **5.1 Innate immune system dysfunction in malnutrition**

Malnutrition affects the primary physical defensive barrier of the body. Thinning of dermis and reduced collagen levels are seen in animal models of PEM. In marasmic mice, thinning of the epidermis, with decreased hydration of stratum corneum, and decreased epidermal proliferation are seen. Wound healing is delayed and there is delayed wound contraction. Increased infiltration of wound site with inflammatory cells, decreased laying down of collagen, oedema of the extracellular matrix and altered neovascularisation are seen. Though skin changes like oedema and 'flaky paint dermatosis' and desquamation are common in kwashiorkor, there is no definite clinical proof to associate these changes with decreased immunity.

The changes in gut mucosa are more dramatic and have greater clinical consequences. These have been discussed above in environmental enteric dysfunction. At this juncture it suffices to say that gastric acid secretion is decreased in severe malnutrition, and gut permeability to bacteria is increased. In the oral cavity flow of saliva is reduced. Vitamin A deficiency reduces differentiation of epithelial cells in the skin, cornea and respiratory, urogenital and gastrointestinal tracts. This compromised epithelial barrier makes bacterial and viral invasion easy. Retinoic acid deficiency can alter gut mucosal barrier function. There is a marked reduction of type 3 innate lymphoid cells (ILC3) in gut mucosa of mice with vitamin A deficiency resulting in decreased production of IL-17 and Il-22 and increased susceptibility to bacterial infections. Concomitantly, there is an expansion of type 2 innate lymphoid cells (ILC2). These cells secrete IL-13 which causes goblet cell hyperplasia, increased mucus production and an increased resistance to gut helminths. Zinc reduces biofilm formation and decreases expression of virulence and adherence factors of entero-aggregative *Escherichia coli*. This may be one of the reasons for frequent diarrhoeal illnesses in malnourished children. A double-blinded randomised placebo controlled trial on zinc supplementation in children between the ages of 6 and 30 months in Delhi showed reduction in frequency, severity and duration of diarrhoea disease in the zinc-supplemented group [117].

Acute-phase reactant synthesis seems unaffected by malnutrition. C-reactive protein (CRP) rise is similar in normal and malnourished children when faced with an infectious challenge [118]. In contrast the so-called negative phase reactants like transferrin, pre-albumin, fibronectin and α2-HS glycoprotein are consistently decreased in malnutrition and do not rise adequately during an infectious challenge [119]. Complement levels are decreased in severe malnutrition. This is due to reduced synthesis in the liver and also increased consumption in the periphery. This is most marked in kwashiorkor. Reduction in C3 levels in malnutrition has been consistently reported from other studies as well [120].

As it has already been mentioned, there is no reduction in the total number of leucocytes in the peripheral blood. But chemotaxis and microbicidal activity of neutrophils are decreased in children with PEM. This may partly be due to decreased lysosomal enzyme synthesis and reduced glycolytic activity. Vitamin A is important for neutrophil maturation. Neutrophils in retinoic acid-deficient mouse models show impaired chemotaxis, phagocytosis and generation of reactive oxygen species. Vitamin A deficiency also decreases number and function of NK cells. Vitamin C deficiency in animals decreases apoptosis of neutrophils and results in their decreased clearance. Vitamin C deficiency exaggerates inflammation and retards its resolution in mouse model of sterile inflammation. Administration of vitamin C attenuates lung, kidney and liver injury in murine models of intraabdominal sepsis and lethal LPS administration. The salutary effects of vitamin C in the lung include reduced pro-inflammatory response, increased epithelial barrier

**41**

*Malnutrition in HIV/AIDS: Aetiopathogenesis DOI: http://dx.doi.org/10.5772/intechopen.90477*

inside macrophages [123].

function, increased alveolar fluid clearance and decreased coagulopathy. Zinc modulates respiratory bursts in neutrophils. Moderate to severe malnutrition does not lead to reduction in absolute number of natural killer (NK) cells in children, but their function is depressed. Iron deficiency impairs macrophage function. Intracellular iron activates nuclear factor kB (NF kB). NfkB is responsible for exerting a restraint on reactive oxygen species and c-Jun-N-terminal kinase (JNK) signalling. These signalling pathways are crucial for antagonism of programmed cell death (PCD) induced by pro-inflammatory cytokines. Hypoxia-inducible factor-1α (HIF-1α) is an iron-dependent transcription factor that promotes synthesis of antimicrobial peptides by macrophages. Hence iron deficiency can lead to apoptosis

of macrophages and also blunt their anti-microbial activity [121, 122].

Folate deficiency in rats is associated with reduced number of neutrophils and eosinophils. Zinc regulates release of pro-inflammatory cytokines like IL-1β, IL-6 and IFN-α by innate immune cells, and its deficiency leads to reduced synthesis of Th-1 cytokines. Selenium exerts its antioxidant activity via selenoproteins. They regulate pro-inflammatory mediators via mitogen-activated protein kinase and peroxisome proliferator-activated receptor-γ. Genetic knockout of selenoprotein genes in mice leads to impaired migration of macrophages and neutrophils and reduced phagocyte oxidative burst. Vitamin D plays a crucial role in macrophage function. Its deficiency increases risk of active TB and also increases risk of relapse of TB in both HIV-uninfected and HIV-positive individuals. The primary action of vitamin D is exerted via the vitamin D receptors on macrophages. There is a supplementary action via toll-like receptor signalling. Vitamin D leads to increased production of cathelicidin and β2-defensin which increase secretion of pro-inflammatory cytokines, induce anti-tuberculous autophagy and restrict growth of mycobacteria

In severe malnutrition, dendritic cell numbers are reduced in peripheral blood and in lymphoid tissues. A study from Zambia showed reduced numbers of DCs and also impaired DC maturation and impaired ability of DCs to stimulated T-cell proliferation in the face of endotoxaemia. The DC function normalised with nutritional therapy [115]. Murine models with deficiency of protein, iron and zinc have shown reduction in number resident DCs in lymph nodes. These DCs also showed dysregulation of DC chemoattractants during inflammation. PEM grossly impairs antigen-presenting capacity and T-cell activation ability of DCs. Vitamin A, as retinoic acid, is vital for DC function. When there is inflammation, retinoic acid accelerates maturation and antigen-presenting capacity of DCs. Dendritic cells also store and release retinoic acid to act on other immune cells. Vitamin D plays an important role in regulation of DC function and exerts an anti-inflammatory role. It

Malnutrition does not affect the total number of lymphocytes in peripheral blood. The total levels of immunoglobulins IgG and IgM in blood and secretory IgA in duodenal fluid and urine are unaltered. When malnutrition in children is compounded by a severe respiratory or intestinal bacterial infection, the number of B cells is reduced when compared to infected but well-nourished children. B lymphocyte function is preserved in PEM, but the profile of secreted immunoglobulins (Igs) and specific antibody-mediated immune responses are altered. Levels of Th1-type immunoglobulins (IgG2a and IgG3) are unaltered, those of tTh2-type Igs (IgG1 and IgE) are raised and those of secretory IgA are reduced. Severe protein malnutrition leads to decreased levels of secretory IgA in tears and saliva. Zinc deficiency depletes cells of B-cell lineage in the bone marrow. Vitamin A-deficient

retards DC maturation, antigen presentation and T-cell priming.

**5.2 Malnutrition and adaptive immune system**

#### *Malnutrition in HIV/AIDS: Aetiopathogenesis DOI: http://dx.doi.org/10.5772/intechopen.90477*

*Nutrition and HIV/AIDS - Implication for Treatment, Prevention and Cure*

Malnutrition affects the primary physical defensive barrier of the body. Thinning of dermis and reduced collagen levels are seen in animal models of PEM. In marasmic mice, thinning of the epidermis, with decreased hydration of stratum corneum, and decreased epidermal proliferation are seen. Wound healing is delayed and there is delayed wound contraction. Increased infiltration of wound site with inflammatory cells, decreased laying down of collagen, oedema of the extracellular matrix and altered neovascularisation are seen. Though skin changes like oedema and 'flaky paint dermatosis' and desquamation are common in kwashiorkor, there is no definite clinical proof to associate these changes with decreased

The changes in gut mucosa are more dramatic and have greater clinical consequences. These have been discussed above in environmental enteric dysfunction. At this juncture it suffices to say that gastric acid secretion is decreased in severe malnutrition, and gut permeability to bacteria is increased. In the oral cavity flow of saliva is reduced. Vitamin A deficiency reduces differentiation of epithelial cells in the skin, cornea and respiratory, urogenital and gastrointestinal tracts. This compromised epithelial barrier makes bacterial and viral invasion easy. Retinoic acid deficiency can alter gut mucosal barrier function. There is a marked reduction of type 3 innate lymphoid cells (ILC3) in gut mucosa of mice with vitamin A deficiency resulting in decreased production of IL-17 and Il-22 and increased susceptibility to bacterial infections. Concomitantly, there is an expansion of type 2 innate lymphoid cells (ILC2). These cells secrete IL-13 which causes goblet cell hyperplasia, increased mucus production and an increased resistance to gut helminths. Zinc reduces biofilm formation and decreases expression of virulence and adherence factors of entero-aggregative *Escherichia coli*. This may be one of the reasons for frequent diarrhoeal illnesses in malnourished children. A double-blinded randomised placebo controlled trial on zinc supplementation in children between the ages of 6 and 30 months in Delhi showed reduction in frequency, severity and

duration of diarrhoea disease in the zinc-supplemented group [117].

consistently reported from other studies as well [120].

Acute-phase reactant synthesis seems unaffected by malnutrition. C-reactive protein (CRP) rise is similar in normal and malnourished children when faced with an infectious challenge [118]. In contrast the so-called negative phase reactants like transferrin, pre-albumin, fibronectin and α2-HS glycoprotein are consistently decreased in malnutrition and do not rise adequately during an infectious challenge [119]. Complement levels are decreased in severe malnutrition. This is due to reduced synthesis in the liver and also increased consumption in the periphery. This is most marked in kwashiorkor. Reduction in C3 levels in malnutrition has been

As it has already been mentioned, there is no reduction in the total number of leucocytes in the peripheral blood. But chemotaxis and microbicidal activity of neutrophils are decreased in children with PEM. This may partly be due to decreased lysosomal enzyme synthesis and reduced glycolytic activity. Vitamin A is important for neutrophil maturation. Neutrophils in retinoic acid-deficient mouse models show impaired chemotaxis, phagocytosis and generation of reactive oxygen species. Vitamin A deficiency also decreases number and function of NK cells. Vitamin C deficiency in animals decreases apoptosis of neutrophils and results in their decreased clearance. Vitamin C deficiency exaggerates inflammation and retards its resolution in mouse model of sterile inflammation. Administration of vitamin C attenuates lung, kidney and liver injury in murine models of intraabdominal sepsis and lethal LPS administration. The salutary effects of vitamin C in the lung include reduced pro-inflammatory response, increased epithelial barrier

**5.1 Innate immune system dysfunction in malnutrition**

**40**

immunity.

function, increased alveolar fluid clearance and decreased coagulopathy. Zinc modulates respiratory bursts in neutrophils. Moderate to severe malnutrition does not lead to reduction in absolute number of natural killer (NK) cells in children, but their function is depressed. Iron deficiency impairs macrophage function. Intracellular iron activates nuclear factor kB (NF kB). NfkB is responsible for exerting a restraint on reactive oxygen species and c-Jun-N-terminal kinase (JNK) signalling. These signalling pathways are crucial for antagonism of programmed cell death (PCD) induced by pro-inflammatory cytokines. Hypoxia-inducible factor-1α (HIF-1α) is an iron-dependent transcription factor that promotes synthesis of antimicrobial peptides by macrophages. Hence iron deficiency can lead to apoptosis of macrophages and also blunt their anti-microbial activity [121, 122].

Folate deficiency in rats is associated with reduced number of neutrophils and eosinophils. Zinc regulates release of pro-inflammatory cytokines like IL-1β, IL-6 and IFN-α by innate immune cells, and its deficiency leads to reduced synthesis of Th-1 cytokines. Selenium exerts its antioxidant activity via selenoproteins. They regulate pro-inflammatory mediators via mitogen-activated protein kinase and peroxisome proliferator-activated receptor-γ. Genetic knockout of selenoprotein genes in mice leads to impaired migration of macrophages and neutrophils and reduced phagocyte oxidative burst. Vitamin D plays a crucial role in macrophage function. Its deficiency increases risk of active TB and also increases risk of relapse of TB in both HIV-uninfected and HIV-positive individuals. The primary action of vitamin D is exerted via the vitamin D receptors on macrophages. There is a supplementary action via toll-like receptor signalling. Vitamin D leads to increased production of cathelicidin and β2-defensin which increase secretion of pro-inflammatory cytokines, induce anti-tuberculous autophagy and restrict growth of mycobacteria inside macrophages [123].

In severe malnutrition, dendritic cell numbers are reduced in peripheral blood and in lymphoid tissues. A study from Zambia showed reduced numbers of DCs and also impaired DC maturation and impaired ability of DCs to stimulated T-cell proliferation in the face of endotoxaemia. The DC function normalised with nutritional therapy [115]. Murine models with deficiency of protein, iron and zinc have shown reduction in number resident DCs in lymph nodes. These DCs also showed dysregulation of DC chemoattractants during inflammation. PEM grossly impairs antigen-presenting capacity and T-cell activation ability of DCs. Vitamin A, as retinoic acid, is vital for DC function. When there is inflammation, retinoic acid accelerates maturation and antigen-presenting capacity of DCs. Dendritic cells also store and release retinoic acid to act on other immune cells. Vitamin D plays an important role in regulation of DC function and exerts an anti-inflammatory role. It retards DC maturation, antigen presentation and T-cell priming.

#### **5.2 Malnutrition and adaptive immune system**

Malnutrition does not affect the total number of lymphocytes in peripheral blood. The total levels of immunoglobulins IgG and IgM in blood and secretory IgA in duodenal fluid and urine are unaltered. When malnutrition in children is compounded by a severe respiratory or intestinal bacterial infection, the number of B cells is reduced when compared to infected but well-nourished children. B lymphocyte function is preserved in PEM, but the profile of secreted immunoglobulins (Igs) and specific antibody-mediated immune responses are altered. Levels of Th1-type immunoglobulins (IgG2a and IgG3) are unaltered, those of tTh2-type Igs (IgG1 and IgE) are raised and those of secretory IgA are reduced. Severe protein malnutrition leads to decreased levels of secretory IgA in tears and saliva. Zinc deficiency depletes cells of B-cell lineage in the bone marrow. Vitamin A-deficient

mice show a poor IgG response which is reversible by vitamin A supplementation. Vitamin A-deficient mice also have decreased number of IgA-secreting plasma cells in their Peyer's patches [123, 124].

Levels of CD4+ and CD8+ cells in peripheral blood remain unaltered in malnourished children hospitalised with serious infections. But there is a decrease in the number of CD4+ CD45RO+ memory T cells and effector T cells (CD4+ CD62L− and CD8+ CD28−) in severe malnutrition. Th1 cytokines required for Th1 differentiation (IL-7, IL-12, IL-18 and IL-21) and function (IL-2 and IFN-γ) are reduced in peripheral blood mononuclear cells of children with malnutrition and severe infection. In the same children, an overexpression of Th2 cytokines (IL-4 and IL-10) and increased apoptosis of CD3+ T cells is noted. The ability of T cells to respond to an inflammatory stimulus is also altered. There is an impaired antigen-specific T-cell response (decreased CD8+ cells and decreased IL-2 production by CD4+ cells), but antigen-specific antibody production is unimpaired. Proliferative response to phytohaemagglutinin is reduced. Delayed-type hypersensitivity response is also impaired in severe malnutrition. Zinc is required for Th1 differentiation and Th1 responses. It increases expression of IL-2, IFN-γ and IL-2Rb β2. Zinc deficiency therefore results in a reduction of CD4/CD8 ratio and levels of Th1 cytokines. Selenium deficiency adversely affects CD4+ T-cell proliferation, activation and function. The production of IL-2 and expression of IL-2 receptor are both reduced, and there is impaired mobilisation of calcium. Retinoic acid acts on naive T cells and promotes expression of gut-homing receptors, differentiation into Th2 phenotype and T-regulatory cells especially in the gut mucosa. It also inhibits maturation to Th1 phenotype or Th17 cells. RA activates B cells in mucosa and GALT to transform into IgA+ antibody secreting cells (ASC). Hence RA deficiency can seriously impair gut mucosal immunity. Due to its influence on effector T-cell function, vitamin A deficiency can lead to inadequate immune response to some vaccines. Vitamin A supplementation has been shown to lead to 20–30% reduction in all-cause mortality and reduction of incidence and severity diarrhoea diseases and measles [125, 126].

The effect of HIV infection on immune systems mirrors that of malnutrition in most aspects with just a few key differences. Natural killer cell activity and complement activity are increased in HIV infection. There is an increased secretion of pro-inflammatory cytokines (IL-1β, TNF-α, INF-γ and IL-6, IL-8 and soluble IL-2 receptors) and reduction of anti-inflammatory cytokines (IL-1 receptor antagonist, IL-4, IL-10 and IL-13). The chronic inflammatory state is also a hypercatabolic one and leads to increased mobilisation of amino acids from skeletal muscles that are further used for gluconeogenesis in the liver. TNF-α and IFN-γ also suppress appetite leading to decreased food intake. Hypercatabolism and decreased diet lead to malnutrition and HIV wasting. The combination of HIV and malnutrition aggravates reduction of CD4 and CD8 T cells, impairs bactericidal function of neutrophils and macrophages, impairs delayed-type hypersensitivity response and blunts antibody response to immunisation [97].

HIV infection also has a direct impact on nutrition. Studies have shown that among asymptomatic HIV-positive children, the rates of protein, carbohydrate and fat malabsorption are 30–60, 32 and 30%, respectively [127, 128]. Increased protein turnover occurs to cater for proliferation of neutrophils, fibroblasts and lymphocytes, production of immunoglobulins and acute-phase reactants and increased urinary nitrogen loss. This mainly comes from increased skeletal muscle breakdown and increased hepatic protein synthesis. Other metabolic changes that occur include elevated hepatic fatty acid synthesis, decreased peripheral lipoprotein lipase activity, hypertriglyceridemia, increased gluconeogenesis, insulin resistance and hyperglycaemia. There is redistribution of body stores of iron and zinc, with both being mobilised to the liver. This along with inadequate dietary intake leads to iron and

**43**

**Author details**

**6. Conclusion**

Vangal K. Sashindran1

\* and Rajneesh Thakur2

\*Address all correspondence to: vksashindran@gmail.com

provided the original work is properly cited.

1 Internal Medicine, Armed Forces Medical Services, India, Prayagraj, India

2 Internal Medicine, Armed Forces Medical Services, India, Kanpur, India

© 2020 The Author(s). Licensee IntechOpen. 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,

*Malnutrition in HIV/AIDS: Aetiopathogenesis DOI: http://dx.doi.org/10.5772/intechopen.90477*

mitigate NIADS and improve HIV clinical outcomes.

zinc deficiencies. The pro-inflammatory state also leads to increased consumption of vitamins A, C and E which serve as antioxidants. There is reduction in levels of glutathione which is the principal intracellular antioxidant compound. Deficiencies of micronutrients like selenium, zinc, manganese and copper affect function of many key antioxidant enzymes. Deficiency of antioxidants leads to increased oxidative stress which triggers T-cell apoptosis and also enhances HIV replication [129].

Malnutrition depresses all aspects of immune function. HIV infection can lead to wasting and malnutrition by a complex interplay of aetiological factors. This malnutrition compounds immunodeficiency of AIDS and accelerates progression of disease and increases risk of mortality. Addressing nutrition right from the time of HIV diagnosis is a good strategy. Judicious and monitored nutritional therapy can

*Malnutrition in HIV/AIDS: Aetiopathogenesis DOI: http://dx.doi.org/10.5772/intechopen.90477*

zinc deficiencies. The pro-inflammatory state also leads to increased consumption of vitamins A, C and E which serve as antioxidants. There is reduction in levels of glutathione which is the principal intracellular antioxidant compound. Deficiencies of micronutrients like selenium, zinc, manganese and copper affect function of many key antioxidant enzymes. Deficiency of antioxidants leads to increased oxidative stress which triggers T-cell apoptosis and also enhances HIV replication [129].
