HIV infection and the gastrointestinal tract
Kotler, Donald P
From the Gastrointestinal Division, Department of Medicine, St Luke's Roosevelt Hospital Center, College of Physicians and Surgeons, Columbia University, New York, USA
Received 1 December, 2003
Accepted 1 December, 2003
Correspondence to Donald P. Kotler, MD, GI Division/S&R 1301, St Luke's Roosevelt Hospital Center, 1111 Amsterdam Avenue, New York, NY 10025, USA. Tel: +1 212 523 3670; fax: +1 212 523 3678; e-mail: firstname.lastname@example.org
Gastrointestinal mucosa and other mucous membranes occupy a unique anatomical niche: the interface between a sterile, internal environment and a contaminated, external environment. There is a polarity in epithelia that is different from all other tissues, in which one side of the epithelial cell faces ‘self’, whereas the other side faces ‘non-self’. Epithelial cells require intimate contact with the external environment in order to carry out their primary functions, e.g. nutrient absorption in the intestine, gas exchange in the lungs, etc. The need for such contact with the external environment makes mucous membranes inherently vulnerable to infectious organisms, because of the lack of a strong physical barrier. The problem is compounded by a large surface area that characterizes most mucous membranes. For example, in the intestine, adaptations such as mucosal folds, villi and microvilli lead to a 600-fold increase in surface area beyond that present in a simple tube.
An elaborate system has evolved to protect the gastrointestinal tract from pathogens, which is part of a common mucosal immune system. The afferent and efferent limbs of the mucosal immune system are anatomically separate, but intermingled. The afferent limb represents the cell populations and structures involved in the production of an immune response, e.g. antigen presentation and lymphoproliferation, whereas the efferent limb involves the cell populations and structures directly involved in the immune response, e.g. antibody producing and cytotoxic mononuclear cells. The afferent system includes discrete lymphoid follicles (Fig. 1), which are overlaid by a follicle-associated epithelium, including microfold, or M cells. M cells are specially adapted epithelial cells, without long microvilli or overlying mucus, which promote the transcellular transit of particulate antigens to antigen-presenting macrophages that lie immediately beneath the cells, and then to mucosal lymphoid follicles (Fig. 2). Further maturation of cells occurs during migration through mesenteric lymphoid follicles and the systemic circulation, via the thoracic duct. After further maturation in the spleen and elsewhere, lymphoid cells return to the intestinal mucosa. The lymphoid cells in the efferent compartments, the epithelial layer, and the lamina propria are scattered diffusely throughout the intestine, in keeping with the defensive function along the length of the intestine. Differences in subpopulations of lymphoid cells, and in immune functions, are found in the epithelial layer and in the lamina propria.
Mucosal immunity has many homologies to systemic immunity, and several distinctions. Although many consider the intestinal mucosa to be physiologically inflamed, it is quite uninflamed, despite microbial provocation. Secretory immunoglobulins and the process of immune exclusion from the internal environment, as well as the lack of complement activation by IgA, reflect the non-inflammatory tone of the intestines. Importantly, inflammation impairs mucosal function by several mechanisms.
Intestinal mucosa has long been known to be a target for HIV and related viruses, based upon the route of exposure and other aspects of mucosal immunology (Fig. 3). The aim of this review is to document the progress in the field of HIV infection in the gastrointestinal tract over the past few years. Whereas new information is partly recapitulation of earlier work, discussion will concentrate on recently published studies.
Early conceptions of HIV transmission were that it occurred through mucosal trauma or other breaches in the physical barriers of the genital epithelium or perineal skin. However, infection also occurs after atraumatic inoculation of mucous membranes. The route could be through epithelial cells, between epithelial cells, or by the normal route for particulate sampling, the M cells. Initial studies, using rabbit ileum containing Peyer's patches including M cells, showed the penetration of HIV from lumen to lamina propria (Fig. 4) . Fotopoulous and colleagues  constructed polarized monolayers using an intestinal cell line (Caco-2), which resembles M cells when grown in the presence of B lymphocytes. A lymphocyte tropic, or X4 HIV strain, crossed the M-cell monolayers and infected underlying CD4 target cells. Transport required both lactosyl cerebroside and CXCR4 receptors, which were expressed on the apical surface of the Caco-2 cells. Antibodies specific for each receptor blocked transport. In contrast, a monotropic, or R5, strain was unable to cross the M-cell monolayers and infect underlying monocytes. Caco-2 cells do not express CCR5 under normal circumstances, but transfection of these cells with CCR5 complementary DNA enabled transport of R5 virus, demonstrating that HIV-1 transport across M cells is receptor mediated. However, the investigators also studied human tissues and found that follicle-associated epithelium expressed galactosyl ceramide and CCR5, but not CXCR4.
Van de Perre  and Meng and colleagues  also investigated viral penetration in the small intestine. They noted that the upper gastrointestinal tract is a principal route of HIV-1 entry in cases of mother-to-child transmission . The phenotype of the newly acquired virus is predominantly R5 and not X4, although both viruses are frequently inoculated onto the mucosa. Primary cultures of jejunal epithelial cells expressed galactosyl ceramide and CCR5, but not CXCR4, and the cells transferred R5, but not X4, viruses to target cells . Transfer was not inhibited by the fusion inhibitor T-20 (Enfuvirtide; Roche Pharmaceuticals, Nutley, NJ, USA), but was substantially reduced by colchicine and by low temperature, consistent with endocytotic uptake and microtubule-dependent transcytosis by the epithelial cells.
Bouhlal and colleagues  demonstrated that infection of HT29 cells was enhanced twofold when semen was added to HIV before incubating with epithelial cell cultures. The enhancing effect of semen was complement dependent, as evidenced by blockage of the generation of C3a/C3adesArg in semen by heat or ethylenediamine tetraacetic acid treatment, and by the suppression of semen-dependent enhancement with monoclonal antibodies directed to complement receptor 3. The investigators concluded that the activation of complement in semen may facilitate the infection of epithelial or lamina propria cells.
Hocini and Bomsel  tested the specificity of transcytosis across the epithelium by incubating with secretory IgA or IgG, purified from colostrum, and showed that they inhibited both transcytosis and the infection of target cells. Devito and colleagues  studied highly HIV-exposed but uninfected individuals, and also found that IgA from plasma, saliva, and mucosal secretions inhibited HIV-1 transcytosis across an epithelial cell membrane system, whereas IgA isolated from low-risk, healthy control subjects did not.
Compartmentalization of viral infection
The results of several studies have suggested that there is compartmentalization of viral infection between blood and mucosa. Selective quasispecies transmission after systemic or mucosal exposure of macaques to SIV was studied by Nieldez and colleagues . Macaques were inoculated intravenously, intrarectally, or intravaginally with SIVmac251. The patterns of virological and immunological events differed substantially between vaginally inoculated animals, who had transient viremia and late seroconversion, and intravenously or intrarectally inoculated monkeys, who had persistent viremia and early seroconversion. Analysis of the envelope gene nucleotide sequences revealed specific viral variants that were associated with vaginal transmission. Couedel-Courteille and colleagues  also studied rectal infection in rhesus macaques inoculated with SIVmac251. Infection was established initially in certain paracolic lymph node chains draining the rectum. Specific sequences of infection within lymphoid follicles and among different lymphoid chains were noted and contrasted with those in intravenously infected animals.
The compartmentalization of infection is partly caused by viral factors. Harouse and colleagues  demonstrated different pathogenic sequelae from infection by X4 and R5 SHIV-derived viruses. The R5 virus clone caused a dramatic loss of intestinal CD4 T cells followed by a gradual depletion in peripheral CD4 T cells, whereas infection with the X4 clone caused a profound loss in peripheral T cells that was not paralleled in the intestine.
Other evidence of viral compartmentalization was provided by Poles and colleagues , who compared genotypic and phenotypic resistance patterns of HIV-1 RNA isolated from colonic mucosa, plasma and peripheral blood mononuclear cells. There was high concordance in detecting mutations in the reverse transcriptase and protease genes as well as in phenotypic resistance patterns. However, different genotypic features in isolates from plasma and the other tissue compartments were observed in some cases.
Cell targets for HIV infection
Several groups have examined cell targets in the intestine. Initial studies documented the ability of HIV to replicate in intestinal epithelial cell lines [12,13], as well as the presence of HIV DNA, RNA, and protein antigens in intestinal mucosa from clinical specimens [14–16], although convincing evidence of the high-level replication of HIV in epithelial cells in clinical samples is lacking. Smith et al.  and Meng et al.  examined purified lamina propria lymphocytes and macrophages from the normal human small intestine. Lamina propria lymphocytes expressed CD4, CCR5, and CXCR4. In contrast, lamina propria macrophages expressed CD4 but neither CCR5 nor CXCR4 (Fig. 5). Intestinal lymphocytes supported replication by R5 and X4 isolates of HIV-1, whereas lamina propria macrophages were permissive to neither. Regulated upon activation: normal T cell expressed/secreted (RANTES), macrophage inflammatory protein (MIP) 1α, and MIP-1β inhibited the infection of intestinal lymphocytes by an R5 viral isolate, suggesting that R5 infection was mediated by CCR5. The investigators concluded that resident lamina propria lymphocytes, not macrophages, are the target mononuclear cell for HIV-1 infection in intestinal mucosa. Lapenta and colleagues  also showed that human lamina propria lymphocytes, in contrast to autologous peripheral blood lymphocytes, are permissive to both X4 and R5 strains. Anton and colleagues  demonstrated enhanced levels of functional CCR5 and CXCR4 on human mucosal T lymphocytes obtained from intestinal biopsies, compared with peripheral blood lymphocytes. In another study, Poles and colleagues  showed that a greater susceptibility of mucosal lymphocytes to HIV-1 infection compared with peripheral blood lymphocytes was associated with a greater expression of chemokine receptors.
These data, in combination with studies of viral penetration, suggest that the primary infection of intestinal mucosa involves R5 virus, which crosses the epithelial layer by several mechanisms, including transcytosis across M cells, and infects lamina propria lymphocytes. Infection of intestinal mononuclear cells with X4 virus is also possible, but by a different, internal route, the systemic circulation, as opposed to the external environment.
The topic of immunopathogenesis involves two separate but overlapping phenomena, the loss of lymphoid cells and the loss of immune function. Several studies of clinical specimens, using immunohistochemistry or flow cytometry, showed a disproportionate early loss of CD4 lymphocytes in lamina propria, compared with peripheral blood , with greater relative losses of CD4 lymphocytes from lamina propria than from lymphoid follicles, at least early in the disease course . Veazey and colleagues  showed that primary SIV infection of rhesus monkeys with SIVmac239 resulted in profound and selective depletion of CD4 T cells in the intestine within days of infection, before any changes occurred in peripheral lymphoid tissues. Similar findings were reported by Kewenig and colleagues . Fackler et al.  and Schmidt and colleagues  found a homogenous distribution of HIV proviral DNA in peripheral blood and intestinal mucosa but more p24 production in lamina propria. Their results suggested that mucosal HIV production is upregulated at the transcriptional or translational level.
The effect of HIV infection upon cell death by apoptosis has also been studied. There appear to be many more apoptotic cells than HIV-infected cells, suggesting that uninfected cells may undergo apoptosis as innocent bystanders, as suggested in peripheral lymph nodes . Boirivant and colleagues  showed that the preincubation of normal lamina propria lymphocytes, isolated from surgical specimens, with HIV-1 gpl20 increased the number of apoptotic cells observed during subsequent CD2 pathway stimulation. The process was mediated by Fas/Fas ligand interaction and was related to the induction of Fas ligand messenger RNA by gpl20. Extracellular HIV-1 gp120 could thus contribute to the depletion of non-infected lamina propria T cells, independent of direct viral infection.
In other studies, Mattapallil and colleagues  studied intraepithelial lymphocytes in primary experimental SIV infection, and documented an early expansion of interferon-producing CD8αβ T cells, but depletion of resident CD8αα T cells, the cell type that includes intraepithelial CD4 cells. The same group showed an increase in the expression of RANTES, but no relationship between RANTES expression and viral suppression . Talal and colleagues  studied the effect of HIV-1 infection on mucosal lymphocyte proliferation and activation in chronically infected and control subjects. The number of proliferating cells was increased in the intestine, whereas the level of activation as well as markers of mucosal trafficking were no different from controls. One possible reason for this unexpected result is that the HIV-infected subjects were being successfully treated with antiretroviral agents. Studies of infected individuals in the absence of antiretroviral therapy might help clarify the issue.
The role of mucosal inflammation in the pathogenesis of immune deficiency is a long-standing uncertainty. In past studies, we showed correlations among clinical symptoms, histopathological changes on rectal biopsy, and mucosal HIV protein contents, but not with the presence of enteric pathogens [33,34]. HIV protein and RNA expression varied during disease progression and were highest in the preclinical stage of the disease. Histopathological studies demonstrated a polymorphous lymphoid infiltration in the intermediate stage of the disease accompanied by the expression of a diverse group of T helper types 1, 2, and pro-inflammatory cytokines. The number of apoptotic cells in the lamina propria varied during disease progression. On the basis of these data, we hypothesized that clinical symptoms and intestinal injury are directly related to the presence of HIV in the mucosa.
Olsson and colleagues  found similar degrees of mucosal inflammation in biopsies from HIV-1-infected subjects, those with inflammatory bowel disease, and from healthy seronegative controls, including the increased expression of CCR5 and CXCR4, RANTES, MIP-1α, and MIP-1β.
HIV enteropathy refers to a poorly defined clinical entity; the name was chosen not for its specificity but rather not to exceed the limit for the number of characters in a manuscript title . Enteropathy is typically invoked to describe an HIV-infected individual with pathogen-negative diarrhea. The pathogenesis of this symptom is not necessarily the same as the pathogenesis of immune deficiency. However, the possibility that it is related directly to local HIV infection has been considered repeatedly. Studies from our laboratory, which examined the effects of 7 days of combination antiretroviral therapy documented statistically significant improvements in semiquantitative estimates of gastrointestinal symptoms, providing indirect proof of an association . However, those studies did not specify just how the presence of HIV in mucosa leads to symptoms.
There are two major lines of investigation into the pathogenesis of diarrhea related to mucosal HIV infection. Studies of intestinal permeability have been performed at the laboratory of Dr Schultzke , who examined the electrical properties of epithelial membranes, either by incorporating a piece of intestine in an electrical circuit, or by establishing an epithelial cell monolayer that reconstructs cell polarity, tight junctions, and normal ion and water fluxes in place of intestinal tissue. Ion flux creates a current that can be quantitated and used to calculate transepithelial electrical resistance. Transepithelial resistance can be altered by changes in transcellular or intercellular (paracellular) permeability; the latter may be caused by the disruption of the tight junctions between cells. Subsequent changes in ion flux affect transepithelial resistance.
Studies of duodenal biopsies showed normal transepithelial resistance in treated, HIV-infected subjects without diarrhea, but decreased resistance in patients with diarrhea . An increase in lactulose flux was noted, suggesting increased intercellular, as opposed to transcellular, permeability. There was no evidence of a change in ion absorption or secretion. The effect was reproduced by incubating a monolayer with supernatants of HIV-infected mononuclear cells . The effect was also reproduced by incubation with tumor necrosis factor, and inhibited by co-incubation with soluble tumor necrosis factor receptors. The investigators concluded that the effect of HIV upon mucosal ion flux was cytokine mediated. It was not clear whether the effect was caused specifically by the disruption of tight junctions or epithelial cell apoptosis, both of which could increase intercellular permeability. To make matters more complicated clinically, some antiviral agents have also been found to affect epithelial cell barrier function .
Clayton and colleagues  explored an alternative explanation for HIV-associated enteropathy. Epithelial cell integrity is partly maintained by the cell's cytoskeleton of microtubules. Microtubule disrupting agents, such as the anti-gout drug, colchicine, affect cytoskeletal structure, increase intestinal permeability and promote diarrhea. Tubulin depolymerization was studied by immunohistochemistry for acetylated tubulin in HIV-infected and control subjects . A decreased staining of acetylated tubulin was seen in small bowel and colonic epithelial cells from HIV-infected subjects, implying microtubular depolymerization and cytoskeletal alterations. The results were similar in HIV-infected subjects with or without AIDS. Treatment with combination antiretroviral therapy for 7 days, as discussed above, led to an increase in the staining of acetylated tubulin, although to levels lower than seen in controls. The authors concluded that some aspect of HIV infection leads to cytoskeletal changes in epithelial cells.
The possibility that HIV directly affects epithelial cells was investigated further. Other studies by Dayanithi and colleagues  and by Maresca and colleagues  showed that incubation of the intestinal cell line, HT-29, with gp120, applied to the basolateral side, led to an increase in cytosolic calcium, which was associated with both tubulin depolymerization and decreased epithelial resistance (Fig. 6). The effect was seen with both monomeric gp120 as well as virus-associated gp120. The receptor through which gp120 promotes these changes was investigated further by Clayton and colleagues . CCR5 or CXCR4 were felt not to be likely targets, because they are expressed on the luminal surface of the epithelial cell, whereas gp120 is more likely to arise from mononuclear cells in the lamina propria, which faces the basal and lateral surfaces. The orphan G protein coupled receptor GPR15/Bob is a co-receptor for HIV and SIV and promotes viral fusion and infection of SIV, although it is an inefficient co-receptor for HIV. This protein was found in homogenates of small bowel and colonic mucosa, lymph nodes, testis, and liver, but not in many other organs. By immunohistochemistry using antibodies to GPR15/Bob, staining was noted in the basal membrane (Fig. 7). Intracellular calcium was examined in cells loaded with Fluo 4, and calcium spikes were obtained with pico-to-nanomolar concentrations of gp120, applied basally. Calcium signaling was inhibited by pertussis toxin, which is a selective G protein inhibitor, and by pretreatment with antibodies to Bob or to galactosyl ceramide, but not by antibodies to CXCR4. Finally, the changes in calcium signaling were found to be HIV strain related. The authors concluded that HIV enteropathy is a pathophysiological consequence of gp120 exposure, and is independent of epithelial cell HIV infection. This hypothesis and that of Stockmann et al.  from the laboratory of Dr Schultzke are not mutually exclusive.
Mucosal immune responses in HIV infection
As with systemic immunity, the clinical consequences of HIV infection become apparent only with severe immune depletion. Intestinal disease caused by protozoa, mycobacteria, or cytomegalovirus is typically seen in patients with peripheral blood CD4 lymphocyte counts less than 100 cell/mm3 . The widespread application of highly active antiretroviral therapy (HAART) has been associated with substantial mucosal immune reconstitution and a marked decrease in the incidence of enteric infections  (see below).
Several studies examined residual mucosal immune function in HIV infection. Scamurra and colleagues  studied the repertoire of immunoglobulin-producing cells in the lamina propria of HIV-infected and control subjects by immunohistochemistry. The density of Ig-producing cells was similar in both groups. Whereas the proportions of IgA-producing cells were lower in both the duodenum and colon from HIV-1-infected patients compared with controls, the density of IgG-producing cells was higher in the colon. These changes paralleled the results of studies of immunoglobulin concentrations in luminal contents. The investigators also evaluated the plasma cell repertoire and noted that it was relatively intact, suggesting that regulatory mechanisms provide sufficient diversity and effective induction and differentiation of mucosal B cells. Schafer and colleagues  studied secretory immunity during the course of SIV infection looking at tissue, serum and saliva. They corroborated the decrease in IgA and the increase in IgG. They were unable to detect SIV-specific IgA, although there was SIV-specific IgG. Incidentally, they also detected enteritis, indirectly, by showing increased albumen concentrations in luminal contents, indicating a breakdown in mucosal barrier function.
In other studies, Carol and colleagues  performed phenotypic and functional analyses of cells isolated from duodenal biopsies and showed normal or elevated spontaneous IFN-γ and IL-4 secretion despite CD4 depletion. Smit-McBride and colleagues  studied the same question by flow cytometry in experimental SIV infection and found the same general results. However, they noted that the interferon-secreting cells in intestinal mucosa were predominantly CD8 cells, and that the loss of interferon-producing CD4 cells was masked by an increase in interferon-producing CD8 cells.
Cytotoxic T lymphocytes (CTL) may be important in anti-HIV host defense. Shacklett and colleagues  isolated HIV-specific CTL from duodenal and rectal biopsy specimens of chronically infected subjects. They were CD8 and MHC class 1 restricted in their function. Schmitz and colleagues , using a chronically infected SIV model, showed that the density of SIV gag-specific CD8 CTL in gastrointestinal mucosa is comparable to that seen systemically in SIVmac-infected rhesus monkeys.
In a proof of principle study, Belyakov and colleagues  used an HIV peptide immunogen given rectally, followed by rectal challenge with an HIV-1 gp160-expressing recombinant vaccinia virus, to demonstrate that resistance to the transmission of HIV-1 gp160-expressing recombinant vaccinia virus can be accomplished by CD8 CTL present at the mucosal site of exposure. The resistance was ablated by depleting CD8 cells in vivo, and required mucosal CTL, as splenic CTL provided no protection. The efficacy was increased by local mucosal delivery of IL-12 with the vaccine. These results imply that the induction of local mucosal CTL may be critical for the success of a vaccine against viruses transmitted through a mucosal route, such as HIV.
In other studies, Wilson and colleagues  identified SIV envelope-specific CTL in the jejunal mucosa of vaginally exposed, seronegative macaques. Animals with the highest CTL response were protected from later colonic challenge. Subsequent vaginal challenge after progesterone treatment produced systemic infection, but with lower plasma viral loads and delayed disease progression. Ahmed and colleagues  studied exposed but seronegative macaques who produced SIV-specific CTL and were resistant to mucosal SIV challenge. Resistant monkeys produced significantly more RANTES and MIP-1α than non-resistant animals. In addition, SIV infection was suppressed by CD8 cell culture supernatants from these monkeys.
HIV-1-specific mucosal IgA antibodies have been reported to correlate with protection in highly exposed but uninfected individuals, but are variably detected. Wright and colleagues  examimed rectal washes from HIV-1-infected and control subjects in a methodological validation study. Total IgA levels did not differ between HIV-1-infected and uninfected groups. HIV-1-specific IgA antibodies were absent in most samples, whereas HIV-1-specific IgG was found in most rectal washes of HIV-1-infected individuals. Schneider and colleagues  also found low levels of HIV-specific IgA antibody production after the short-term culture of duodenal biopsies in vitro.
These data suggest that cell-mediated immunity may be more important than secretory immunity in adaptive immunity to HIV. It should not be surprising that secretory immune function is irrelevant in the face of a chronic infection of the lamina propria. However, antibody-dependent cytotoxicity could be an important element of host defense in the intestine.
Effects of antiretroviral therapy
Relatively few reports of the effects of antiviral treatment in the gastrointestinal tract have been published. We compared the influence of antiretroviral therapy on intestinal mucosa and peripheral blood by performing studies immediately before, and 7 days after starting combination therapy . Many of the patients were antiretroviral naive, most had non-specific gastrointestinal symptoms, and none had detectable enteric infections. Treatment was associated with marked decreases in gastrointestinal symptoms. Similar relative declines in HIV-RNA contents and increases in CD4 lymphocyte counts were found in blood and mucosa (Fig. 8). Treatment was also associated with a fall in the number of apoptotic cells as measured by in-situ labeling, a change that correlated statistically with the change in mucosal viral burden. Talal and colleagues  evaluated the effects of 6 months of HAART in mucosal and peripheral blood mononuclear cells, and found that levels of multiply spliced HIV-1 RNA declined in parallel fashion in peripheral blood and mucosa, implying a decrease in replicating virus in both compartrments. Lampinen and colleagues  examined the effect of antiviral therapy on the detection of HIV RNA and DNA from rectal swabs in men who have sex with men. In that cross-sectional study comparing antiretroviral-treated to non-treated patients, therapy was associated with the suppression of RNA, but not DNA, suggesting latent viral infection in cell reservoirs.
Immune reconstitution in intestinal mucosa after the initiation of HAART can effectively restore mucosal immunity with clinical benefits, including the eradication of opportunistic pathogens such as cryptosporidia and microsporidia . Although the extent of immune reconstitution is substantial, it may, however, be incomplete. For example, Krzysiek and colleagues  studied the expression of CCR5 and the intestinal homing receptor integrin α4β7 on subpopulations of lymphocytes, and found a profound decrease of circulating α4β7+ lymphocytes and CCR5+ memory lymphocytes with non-lymphoid homing potential (CD62L−CD45RO+). This subpopulation remained depleted despite the control of viral replication with antiretroviral treatment. As protective immunity in vivo depends on lymphocytes carrying homing capacity to non-lymphoid tissue, the data suggest that immune dysfunction may persist despite effective antiviral therapy.
Miao and colleagues  examined duodenal lymphocyte densities and mucosal addressin cell adhesion molecule 1 expression in patients receiving HAART. Compared with controls, HAART-naive patients with AIDS had reduced duodenal CD4 T-cell densities, which increased with treatment, especially in patients with enteric infections. Duodenal mucosal addressin cell adhesion molecule 1 expression was elevated in all HAART-naive patients with AIDS at baseline, fell to normal by 6 months in treated patients without enteric infections, but remained elevated in patients with enteric infections.
In summary, intestinal mucosa has long been known as a target for HIV and related viruses. Viral penetration through the epithelium appears to be receptor mediated and limited to CCR5 binding viruses. Viruses that bind CXCR4 may enter the intestinal compartment via the bloodstream. In either event, intestinal lymphocytes appear to be the initial target for HIV. There is compartmentalization of HIV infection, and different sequelae occur based on both viral and host factors. There is often an early and disproportionate loss of CD4 lymphocytes from the mucosal compartment, compared with peripheral blood. The mode of cell death appears to be apoptosis, and may include both infected and non-infected cells. Antiviral immunity includes both humoral and cell-mediated immune function, with the latter appearing more effective. Studies of antiviral therapy have shown that mucosal HIV is as sensitive to suppression as is plasma HIV, and there is a great capacity for immune reconstitution in intestinal mucosa. Finally, HIV may play a direct role in producing intestinal disease. Alterations in mucosal ion fluxes, which are associated with diarrhea, may result either from proinflammatory cytokine release from HIV-infected mononuclear cells in the lamina propria, or from gp120 interactions with certain G proteins expressed on the basolateral membranes of intestinal epithelial cells, leading to cytoskeletal changes and the opening of tight junctions.
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