CCR5- and CXCR4-tropic HIV-1 are equally cytopathic for their T-cell targets in human lymphoid tissue

A rapid decline in T-cell counts and the progression to AIDS is often associated with a switch from CCR5-tropic (R5) HIV-1 to CXCR4-tropic (X4) HIV-1 or R5/X4 HIV-1 variants. Experimental infection with R5 HIV-1 causes less T-cell depletion than infection with X4 or R5/X4 variants in T-cell cultures, in ex vivo infected human lymphoid tissue and in SCID/hu mice, despite similar replication levels. Experimental genetic changes in those sequences in gp120 that transform R5 HIV-1 variants into otherwise isogenic X4 viruses make them highly cytopathic. Thus, it is now believed that R5 variants are less cytopathic for T cells than are X4 variants. However, it is not known why CCR5-mediated HIV-1 infection does not lead to a massive CD4+ T-cell depletion, as occurs in CXCR4-mediated HIV-1 infection. Here we demonstrate that R5 HIV-1 isolates are indeed highly cytopathic, but only for CCR5+/CD4+ T cells. Because these cells constitute only a small fraction of CD4+ T cells, their depletion does not substantially change the total CD4+ T-cell count. These results may explain why the clinical stage of HIV disease correlates with viral tropism.     

Engineering HIV-resistant human CD4+ T cells with CXCR4-specific zinc-finger nucleases

HIV-1 entry requires the cell surface expression of CD4 and either the CCR5 or CXCR4 coreceptors on host cells. Individuals homozygous for the ccr5Δ32 polymorphism do not express CCR5 and are protected from infection by CCR5-tropic (R5) virus strains. As an approach to inactivating CCR5, we introduced CCR5-specific zinc-finger nucleases into human CD4+ T cells prior to adoptive transfer, but the need to protect cells from virus strains that use CXCR4 (X4) in place of or in addition to CCR5 (R5X4) remains. Here we describe engineering a pair of zinc finger nucleases that, when introduced into human T cells, efficiently disrupt cxcr4 by cleavage and error-prone non-homologous DNA end-joining. The resulting cells proliferated normally and were resistant to infection by X4-tropic HIV-1 strains. CXCR4 could also be inactivated in ccr5Δ32 CD4+ T cells, and we show that such cells were resistant to all strains of HIV-1 tested. Loss of CXCR4 also provided protection from X4 HIV-1 in a humanized mouse model, though this protection was lost over time due to the emergence of R5-tropic viral mutants. These data suggest that CXCR4-specific ZFNs may prove useful in establishing resistance to CXCR4-tropic HIV for autologous transplant in HIV-infected individuals.     

R5 human immunodeficiency virus type 1 (HIV-1) replicates more efficiently in primary CD4+ T-cell cultures than X4 HIV-1

In this report, we present evidence that R5 human immunodeficiency virus type 1 (HIV-1) replicates more efficiently in primary CD4+ T cells than X4 HIV-1. By comparing CD3/CD28-costimulated CD4+ T-cell cultures infected by several X4 and R5 HIV-1 strains, we determined that R5-infected CD4+ T cells produce more virus over time than X4-infected CD4+ T cells. In the first comparison, we found that more cells were infected by the X4-tropic strain LAI than by the R5-tropic strain JR-CSF and yet that higher levels of viral production were detected in the R5-infected cultures. The differential viral production was partially due to the severe cytopathic effects of the X4 virus. We also compared cultures infected with the isogenic HIV-1 strains NL4-3 (X4) and 49.5 (R5). We found that fewer cells were infected by the R5 strain, and yet similar levels of viral production were detected in both infected cultures. Cell death played less of a role in the differential viral production of these strains, as the cell viability remained comparable in both X4- and R5-infected cultures over time. The final comparison involved the primary R5-tropic isolate KP1 and the primary dual-tropic isolate KP2. Although both strains infected similar numbers of cells and induced comparable levels of cytopathicity, viral production was considerably higher in the R5-infected culture. In summary, these data demonstrate that R5 HIV-1 has an increased capacity to replicate in costimulated CD4+ T cells compared to X4 HIV-1.     

Contribution of Vpu, Env, and Nef to CD4 down-modulation and resistance of human immunodeficiency virus type 1-infected T cells to superinfection

Human immunodeficiency virus type 1 (HIV-1) utilizes Vpu, Env, and Nef to down-modulate its primary CD4 receptor from the cell surface, and this function seems to be critical for the pathogenesis of AIDS. The physiological relevance of CD4 down-modulation, however, is currently not well understood. In the present study, we analyzed the kinetics of CD4 down-modulation and the susceptibility of HIV-1-infected T cells to superinfection using proviral HIV-1 constructs containing individual and combined defects in vpu, env, and nef and expressing red or green fluorescent proteins. T cells infected with HIV-1 mutants containing functional nef genes expressed low surface levels of CD4 from the first moment that viral gene expression became detectable. In comparison, Vpu and Env had only minor to moderate effects on CD4 during later stages of infection. Consistent with these quantitative differences, Nef inhibited superinfection more efficiently than Vpu and Env. Notably, nef alleles from AIDS patients were more effective in preventing superinfection than those derived from a nonprogressor of HIV-1 infection. Our data suggest that protection against X4-tropic HIV-1 superinfection involves both CD4-independent and CD4-dependent mechanisms of HIV-1 Nef. X4 was effectively down-regulated by simian immunodeficiency virus and HIV-2 but not by HIV-1 Nef proteins. Thus, maximal protection seems to involve an as-yet-unknown mechanism that is independent of CD4 or coreceptor down-modulation. Finally, we demonstrate that superinfected primary T cells show enhanced levels of apoptosis. Accordingly, one reason that HIV-1 inhibits CD4 surface expression and superinfection is to prevent premature cell death in order to expand the period of effective virus production.     

Cell turnover and cell tropism in HIV-1 infection

Early infection with HIV-1 is dominated by CCR5-tropic (R5, non-syncytium-inducing) viruses. The evolution of CXCR4-tropic (X4, syncytium-inducing) viruses occurs later in the infection and is associated with rapid disease progression. Here, we propose that the tropism of X4 viruses for naive CD4+ T cells is disadvantageous in early infection owing to the low division rate of these cells. In healthy individuals, the division rate of memory cells is nearly ten times higher than that of naive cells and thus the memory-cell tropism of R5 viruses could account for their dominance early in infection. As the division rate of naive T cells increases with CD4+ depletion, X4 viruses come to dominate in late disease.     

A novel mechanism for HIV1-mediated bystander CD4+ T-cell death: neighboring dying cells drive the capacity of HIV1 to kill noncycling primary CD4+ T cells

CD4+ T-cell death is a crucial feature of AIDS pathogenesis, but the mechanisms involved remain unclear. Here, we present in vitro findings that identify a novel process of HIV1 mediated killing of bystander CD4+ T cells, which does not require productive infection of these cells but depends on the presence of neighboring dying cells. X4-tropic HIV1 strains, which use CD4 and CXCR4 as receptors for cell entry, caused death of unstimulated noncycling primary CD4+ T cells only if the viruses were produced by dying, productively infected T cells, but not by living, chronically infected T cells or by living HIV1-transfected HeLa cells. Inducing cell death in HIV1-transfected HeLa cells was sufficient to obtain viruses that caused CD4+ T-cell death. The addition of supernatants from dying control cells, including primary T cells, allowed viruses produced by living HIV1-transfected cells to cause CD4+ T-cell death. CD4+ T-cell killing required HIV1 fusion and/or entry into these cells, but neither HIV1 envelope-mediated CD4 or CXCR4 signaling nor the presence of the HIV1 Nef protein in the viral particles. Supernatants from dying control cells contained CD95 ligand (CD95L), and antibody-mediated neutralization of CD95L prevented these supernatants from complementing HIV1 in inducing CD4+ T-cell death. Our in vitro findings suggest that the very extent of cell death induced in vivo during HIV1 infection by either virus cytopathic effects or immune activation may by itself provide an amplification loop in AIDS pathogenesis. More generally, they provide a paradigm for pathogen-mediated killing processes in which the extent of cell death occurring in the microenvironment might drive the capacity of the pathogen to induce further cell death.     

Cannabinoid receptor 2-mediated attenuation of CXCR4-tropic HIV infection in primary CD4+ T cells

Agents that activate cannabinoid receptor pathways have been tested as treatments for cachexia, nausea or neuropathic pain in HIV-1/AIDS patients. The cannabinoid receptors (CB(1)R and CB(2)R) and the HIV-1 co-receptors, CCR5 and CXCR4, all signal via Gαi-coupled pathways. We hypothesized that drugs targeting cannabinoid receptors modulate chemokine co-receptor function and regulate HIV-1 infectivity. We found that agonism of CB(2)R, but not CB(1)R, reduced infection in primary CD4+ T cells following cell-free and cell-to-cell transmission of CXCR4-tropic virus. As this change in viral permissiveness was most pronounced in unstimulated T cells, we investigated the effect of CB(2)R agonism on to CXCR4-induced signaling following binding of chemokine or virus to the co-receptor. We found that CB(2)R agonism decreased CXCR4-activation mediated G-protein activity and MAPK phosphorylation. Furthermore, CB(2)R agonism altered the cytoskeletal architecture of resting CD4+ T cells by decreasing F-actin levels. Our findings suggest that CB(2)R activation in CD4+ T cells can inhibit actin reorganization and impair productive infection following cell-free or cell-associated viral acquisition of CXCR4-tropic HIV-1 in resting cells. Therefore, the clinical use of CB(2)R agonists in the treatment of AIDS symptoms may also exert beneficial adjunctive antiviral effects against CXCR4-tropic viruses in late stages of HIV-1 infection.     

Humanized mice dually challenged with R5 and X4 HIV-1 show preferential R5 viremia and restricted X4 infection of CCR5+CD4+ T cells

CCR5-tropic (R5) immunodeficiency virus type 1 (HIV-1) strains are highly transmissible during the early stage of infection in humans, whereas CXCR4-tropic (X4) strains are less transmissible. This study aimed to explore the basis for early phase R5 and X4 HIV-1 infection in vivo by using humanized mice dually challenged with R5 HIV-1_NLAD8-D harboring DsRed and X4 HIV-1_NL-E harboring EGFP. Whereas R5 HIV-1 replicated well, X4 HIV-1 caused only transient viremia with variable kinetics; however, this was distinct from the low level but persistent viremia observed in mice challenged with X4 HIV-1 alone. Flow cytometric analysis of HIV-1-infected cells revealed that X4 HIV-1 infection of CCR5⁺CD4⁺ T cells was significantly suppressed in the presence of R5 HIV-1. X4 HIV-1 was more cytopathic than R5 HIV-1; however, this was not the cause of restricted X4 HIV-1 infection because there were no significant differences in the mortality rates of CCR5⁺ and CCR5⁻ cells within the X4 HIV-1-infected cell populations. Taken together, these results suggest that restricted infection of CCR5⁺CD4⁺ T cells by X4 HIV-1 (occurring via a still-to-be-identified mechanism) might contribute to the preferential transmission of R5 HIV-1 during the early phase of infection.     

Infection of CD127+ (interleukin-7 receptor+) CD4+ cells and overexpression of CTLA-4 are linked to loss of antigen-specific CD4 T cells during primary human immunodeficiency virus type 1 infection

We recently found that human immunodeficiency virus (HIV)-specific CD4+ T cells express coreceptor CCR5 and activation antigen CD38 during early primary HIV-1 infection (PHI) but then rapidly disappear from the circulation. This cell loss may be due to susceptibility to infection with HIV-1 but could also be due to inappropriate apoptosis, an expansion of T regulatory cells, trafficking out of the circulation, or dysfunction. We purified CD38+++CD4+ T cells from peripheral blood mononuclear cells, measured their level of HIV-1 DNA by PCR, and found that about 10% of this population was infected. However, a small subset of HIV-specific CD4+) T cells also expressed CD127, a marker of long-term memory cells. Purified CD127+CD4+ lymphocytes contained fivefold more copies of HIV-1 DNA per cell than did CD127-negative CD4+ cells, suggesting preferential infection of long-term memory cells. We observed no apoptosis of antigen-specific CD4+ T cells in vitro and only a small increase in CD45RO+CD25+CD127dimCD4+ T regulatory cells during PHI. However, 40% of CCR5+CD38+++ CD4+ T cells expressed gut-homing integrins, suggesting trafficking through gut-associated lymphoid tissue (GALT). Furthermore, 80% of HIV-specific CD4+ T cells expressed high levels of the negative regulator CTLA-4 in response to antigen stimulation in vitro, which was probably contributing to their inability to produce interleukin-2 and proliferate. Taken together, the loss of HIV-specific CD4+ T cells is associated with a combination of an infection of CCR5+ CD127+ memory CD4+ T cells, possibly in GALT, and a high expression of the inhibitory receptor CTLA-4.     

Autophagy and CD4+ T lymphocyte destruction by HIV-1

The first step of HIV-1 infection is mediated by the binding of envelope glycoproteins (Env) to CD4 and two major coreceptors, CCR5 or CXCR4. The HIV-1 strains that use CCR5 are involved in primo-infection whereas those HIV-1 strains that use CXCR4 play a major role in the demise of CD4+ T lymphocytes and a rapid progression toward AIDS. Notably, binding of X4 Env expressed on cells to CXCR4 triggers apoptosis of uninfected CD4+ T cells. We now have just demonstrated that, independently of HIV-1 replication, transfected or HIV-1-infected cells that express X4 Env induce autophagy and accumulation of Beclin 1 in uninfected CD4+ T lymphocytes via CXCR4. Moreover, autophagy is a prerequisite to Env-induced apoptosis in uninfected bystander T cells, and CD4+ T cells still undergo an Env-mediated cell death with autophagic features when apoptosis is inhibited. To the best of our knowledge, these findings represent the first example of autophagy triggered through binding of virus envelope proteins to a cellular receptor, without viral replication, leading to apoptosis. Here, we proposed hypotheses about the significance of Env-induced Beclin 1 accumulation in CD4+ T cell death and about the role of autophagy in HIV-1 infected cells depending on the coreceptor involved.     

Suppression of CCR5- but not CXCR4-tropic HIV-1 in lymphoid tissue by human herpesvirus 6.

HIV-1 infects target cells via a receptor complex formed by CD4 and a chemokine receptor, primarily CCR5 or CXCR4 (ref. 1). Commonly, HIV-1 transmission is mediated by CCR5-tropic variants, also designated slow/low, non-syncytia-inducer or macrophage-tropic, which dominate the early stages of HIV-1 infection and frequently persist during the entire course of the disease. In contrast, HIV-1 variants that use CXCR4 are typically detected at the later stages, and are associated with a rapid decline in CD4+ T cells and progression to AIDS (refs. 2,7-11). Disease progression is also associated with the emergence of concurrent infections that may affect the course of HIV disease by unknown mechanisms. A lymphotropic agent frequently reactivated in HIV-infected patients is human herpesvirus 6 (HHV-6), which has been proposed as a cofactor in AIDS progression. Here we show that in human lymphoid tissue ex vivo, HHV-6 affects HIV-1 infection in a coreceptor-dependent manner, suppressing CCR5-tropic but not CXCR4-tropic HIV-1 replication, as shown with both uncloned viral isolates and isogenic molecular chimeras. Furthermore, we demonstrate that HHV-6 increases the production of the CCR5 ligand RANTES ('regulated upon activation, normal T-cell expressed and secreted'), the most potent HIV-inhibitory CC chemokine, and that exogenous RANTES mimics the effects of HHV-6 on HIV-1, providing a mechanism for the selective blockade of CCR5-tropic HIV-1. Our data suggest that HHV-6 may profoundly influence the course of HIV-1 infection.     

HIV-1 Vpr Accelerates Viral Replication during Acute Infection by Exploitation of Proliferating CD4+ T Cells In Vivo

The precise role of viral protein R (Vpr), an HIV-1-encoded protein, during HIV-1 infection and its contribution to the development of AIDS remain unclear. Previous reports have shown that Vpr has the ability to cause G₂ cell cycle arrest and apoptosis in HIV-1-infected cells in vitro. In addition, vpr is highly conserved in transmitted/founder HIV-1s and in all primate lentiviruses, which are evolutionarily related to HIV-1. Although these findings suggest an important role of Vpr in HIV-1 pathogenesis, its direct evidence in vivo has not been shown. Here, by using a human hematopoietic stem cell-transplanted humanized mouse model, we demonstrated that Vpr causes G₂ cell cycle arrest and apoptosis predominantly in proliferating CCR5⁺ CD4⁺ T cells, which mainly consist of regulatory CD4⁺ T cells (Tregs), resulting in Treg depletion and enhanced virus production during acute infection. The Vpr-dependent enhancement of virus replication and Treg depletion is observed in CCR5-tropic but not CXCR4-tropic HIV-1-infected mice, suggesting that these effects are dependent on the coreceptor usage by HIV-1. Immune activation was observed in CCR5-tropic wild-type but not in vpr-deficient HIV-1-infected humanized mice. When humanized mice were treated with denileukin diftitox (DD), to deplete Tregs, DD-treated humanized mice showed massive activation/proliferation of memory T cells compared to the untreated group. This activation/proliferation enhanced CCR5 expression in memory CD4⁺ T cells and rendered them more susceptible to CCR5-tropic wild-type HIV-1 infection than to vpr-deficient virus. Taken together, these results suggest that Vpr takes advantage of proliferating CCR5⁺ CD4⁺ T cells for enhancing viremia of CCR5-tropic HIV-1. Because Tregs exist in a higher cycling state than other T cell subsets, Tregs appear to be more vulnerable to exploitation by Vpr during acute HIV-1 infection.     

Death of CD4+ T cells from lymph nodes during primary SIVmac251 infection predicts the rate of AIDS progression

Immunological and virological events that occur during the earliest stages of SIV infection are now considered to have a major impact on subsequent disease progression. In the present study, we demonstrate a clear correlation between progression to AIDS and the rate of in vitro CD4+ (but not CD8+) T cell death in lymph nodes. The dying CD4+ T cells were effector memory T cells, which are critical for the immune response to pathogens. However, there was no correlation between the rate of the viral replication within lymph nodes and the extent of Fas ligand-mediated death, despite the increased sensitivity of CD4+ T cells to death in response to recombinant human Fas ligand. CD4+ T cell death was caspase and apoptosis-inducing factor independent but was clearly associated with mitochondrion damage. Interestingly, higher expression levels of the active form of Bak, a proapoptotic molecule involved in mitochondrial membrane permeabilization, were observed in SIV-infected macaques progressing more rapidly to AIDS. Finally, we demonstrated that the strain of SIV we used requires CCR5 and BOB/GRP15 molecules as coreceptors and caused death of unstimulated noncycling primary CD4+ T cells. Altogether, these results demonstrate that CD4+ T cell death occurring early after SIV infection is a crucial determinant of progression to AIDS and that it is mediated by the intrinsic death pathway.     

V3 loop-determined coreceptor preference dictates the dynamics of CD4+-T-cell loss in simian-human immunodeficiency virus-infected macaques

We used experimental infection of rhesus macaques with envelope gp120 V3 loop isogenic simian-human immunodeficiency virus (SHIV) molecular clones to more clearly define the impact of human immunodeficiency virus type 1 coreceptor usage in target cell selectivity and the rates of CD4+-T-cell depletion. Functional assays demonstrate that substitution of the V3 loop of the pathogenic CXCR4-tropic (X4) SHIV(SF33A2) molecular clone with the corresponding sequences from the CCR5-tropic (R5) SHIV(SF162P3) isolate resulted in a switch of coreceptor usage from CXCR4 to CCR5. The resultant R5 clone, designated SHIV(SF33A2(V3)), is replication competent in vivo, infecting two of two macaques by intravenous inoculation with peak viremia that is comparable to that seen in monkeys infected with X4-SHIV(SF33A2). But while primary infection with the X4 clone was accompanied by rapid and significant loss of peripheral and secondary lymphoid CD4+ T lymphocytes, infection with R5-SHIV(SF33A2(V3)) led to only a modest and transient loss. However, substantial depletion of intestinal CD4+ T cells was observed in R5-SHIV(SF33A2(V3))-infected macaques. Moreover, na?ve T cells that expressed high levels of CXCR4 were rapidly depleted in X4-SHIV(SF33A2)-infected macaques, whereas R5-SHIV(SF33A2(V3)) infection mainly affected memory T cells that expressed CCR5. These findings in a unique isogenic system illustrate that coreceptor usage is the principal determinant of tissue and target cell specificity of the virus in vivo and dictates the dynamics of CD4+-T-cell depletion during SHIV infection.     

Engagement of CD28 modulates CXC chemokine receptor 4 surface expression in both resting and CD3-stimulated CD4+ T cells

Optimal CD4+ T cell activation requires the cooperation of multiple signaling pathways coupled to the TCR-CD3 complex and to the CD28 costimulatory molecule. In this study, we have investigated the expression of surface CXC chemokine receptor 4 (CXCR4) in enriched populations of CD4+ T PBL, stimulated with anti-CD3 and anti-CD28 mAbs, immobilized on plastic. Anti-CD3 alone induced a progressive down-regulation of surface CXCR4, accompanied by a significant decline in the entry of the HXB2 T cell line-tropic (X4-tropic) HIV-1 clone in CD4+ T cells. Of note, this effect was strictly dependent on the presence in culture of CD14+ monocytes. On the other hand, anti-CD28 alone induced a small but reproducible increase in the expression of surface CXCR4 as well as in the entry of HXB2 HIV-1 clone in resting CD4+ T cells. When the two mAbs were used in combination, anti-CD28 potently synergized with anti-CD3 in inducing the expression of CD69 activation marker and stimulating the proliferation of CD4+ T cells. On the other hand, anti-CD28 counteracted the CXCR4 down-modulation induced by anti-CD3. The latter effect was particularly evident when anti-CD28 was associated to suboptimal concentrations of anti-CD3. Because CXCR4 is the major coreceptor for the highly cytopathic X4-tropic HIV-1 strains, which preferentially replicate in proliferating CD4+ T cells, the ability of anti-CD28 to up-regulate the surface expression of CXCR4 in both resting and activated CD4+ T cells provides one relevant mechanism for the progression of HIV-1 disease.     

A preponderance of CCR5(+) CXCR4(+) mononuclear cells enhances gastrointestinal mucosal susceptibility to human immunodeficiency virus type 1 infection

The gastrointestinal mucosa harbors the majority of the body's CD4(+) cells and appears to be uniquely susceptible to human immunodeficiency virus type 1 (HIV-1) infection. We undertook this study to examine the role of differences in chemokine receptor expression on infection of mucosal mononuclear cells (MMCs) and peripheral blood mononuclear cells (PBMCs) by R5- and X4-tropic HIV-1. We performed in vitro infections of MMCs and PBMCs with R5- and X4-tropic HIV-1, engineered to express murine CD24 on the infected cell's surface, allowing for quantification of HIV-infected cells and their phenotypic characterization. A greater percentage of MMCs than PBMCs are infected by both R5- and X4-tropic HIV-1. Significant differences exist in terms of chemokine receptor expression in the blood and gastrointestinal mucosa; mucosal cells are predominantly CCR5(+) CXCR4(+), while these cells make up less than 20% of the peripheral blood cells. It is this cell population that is most susceptible to infection with both R5- and X4-tropic HIV-1 in both compartments. Regardless of whether viral isolates were derived from the blood or mucosa of HIV-1-infected patients, HIV-1 p24 production was greater in MMCs than in PBMCs. Further, the chemokine receptor tropism of these patient-derived viral isolates did not differ between compartments. We conclude that, based on these findings, the gastrointestinal mucosa represents a favored target for HIV-1, in part due to its large population of CXCR4(+) CCR5(+) target cells and not to differences in the virus that it contains.     

Altered T cell surface glycosylation in HIV-1 infection results in increased susceptibility to galectin-1-induced cell death

The massive T cell death that occurs in HIV type 1 (HIV-1) infection contributes profoundly to the pathophysiology associated with AIDS. The mechanisms controlling cell death of both infected and uninfected T cells ("bystander" death) are not completely understood. We have shown that HIV-1 infection of T cells results in altered glycosylation of cell surface glycoproteins; specifically, it decreased sialylation and increased expression of core 2 O-glycans. Galectin-1 is an endogenous human lectin that recognizes these types of glycosylation changes and induces cell death of activated lymphocytes. Therefore we studied the possible contribution of galectin-1 in the pathophysiology of AIDS. O-glycan modifications were investigated on peripheral lymphocytes from AIDS patients. Oligosaccharides from CD43 and CD45 of CEM cells latently infected with HIV-1 were chemically analyzed. Consistent with our previous results, we show that HIV-1 infection results in accumulation of exposed lactosamine residues, oligosaccharides recognized by galectin-1 on cell surface glycoproteins. Both latently HIV-1-infected T cell lines and peripheral CD4 and CD8 T cells from AIDS patients exhibited exposed lactosamine residues and demonstrated marked susceptibility to galectin-1-induced cell death, in contrast to control cultures or cells from uninfected donors. The fraction of cells that died in response to galectin-1 exceeded the fraction of infected cells, indicating that death of uninfected cells occurred. Altered cell surface glycosylation of T cells during HIV-1 infection increases the susceptibility to galectin-1-induced cell death, and this death pathway can contribute to loss of both infected and uninfected T cells in AIDS.     

Mechanisms of gastrointestinal CD4+ T-cell depletion during acute and early human immunodeficiency virus type 1 infection

During acute and early human immunodeficiency virus type 1 (HIV-1) infection (AEI) more than 50% of CD4+ T cells are preferentially depleted from the gastrointestinal (GI) lamina propria. To better understand the underlying mechanisms, we studied virological and immunological events within the peripheral blood (PB) and GI tract during AEI. A total of 32 AEI subjects and 18 uninfected controls underwent colonic biopsy. HIV-1 viral DNA and RNA levels were quantified in CD4+ T cells derived from the GI tract and PB by using real-time PCR. The phenotype of infected cells was characterized by using combinations of immunohistochemistry and in situ hybridization. Markers of immunological memory, activation, and proliferation were examined by flow cytometry and immunohistochemistry, and the host-derived cytotoxic cellular response was examined by using immunohistochemistry. GI CD4+ T cells harbored, on average, 13-fold higher HIV-1 viral DNA levels and 10-fold higher HIV-1 RNA levels than PB CD4+ T cells during AEI. HIV-1 RNA was detected in both "activated" and "nonactivated" mucosal CD4+ T cells. A significantly higher number of activated and proliferating T cells were detected in the GI tract compared to the PB, and a robust cytotoxic response (HIV-1 specificity not determined) was detected in the GI tract as early as 18 days postinfection. Mucosal CD4+ T-cell depletion is multifactorial. Direct viral infection likely accounts for the earliest loss of CD4+ T cells. Subsequently, ongoing infection of susceptible CD4+ T cells, along with activation-induced cellular death and host cytotoxic cellular response, are responsible for the persistence of the lesion.