T-cell homeostasis in HIV infection is neither failing nor blind: modified cell counts reflect an adaptive response of the host

'Rinse and Replace': Boosting T Cell Turnover To Reduce HIV-1 Reservoirs

Latent HIV-1 persists indefinitely during antiretroviral therapy (ART) as an integrated silent genome in long-lived memory CD4+ T cells. In untreated infections, immune activation increases the turnover of intrinsically long-lived provirus-containing CD4+ T cells. Those are 'washed out' as a result of their activation, which when coupled to viral protein expression can facilitate local inflammation and recruitment of uninfected cells to activation sites, causing latently infected cells to compete for survival. De novo infection can counter this washout. During ART, inflammation and CD4+ T cell activation wane, resulting in reduced cell turnover and a persistent reservoir. We propose accelerating reservoir washout during ART by triggering sequential waves of polyclonal CD4+ T cell activation while simultaneously enhancing virus protein expression. Reservoir reduction as an adjunct to other therapies might achieve lifelong viral control.

Visceral leishmaniasis in Northeast Brazil: What is the impact of HIV on this protozoan infection?

Background

The aim of this study was to compare cases of Visceral Leishmaniasis (VL) with and without HIV in a state in northeastern Brazil.

Methodology

We performed a comparative study in the state’s referral hospital for infectious/parasitic diseases located in Northeast Brazil between January 2007 and July 2017. The data obtained using this protocol were analyzed with SPSS.

Principal findings

In total, 252 patients were evaluated, including 126 with coincident VL/HIV and 126 with VL alone. Both groups primarily consisted of male patients. The most commonly affected ages were 30–39 years in the coinfected group and 19–29 years in the VL group (p < 0.001). Fever and anorexia (p = 0.001), which were more common in those with VL alone, were frequently observed, while diarrhea, vomiting, bleeding and dyspnea were more common in patients with VL/HIV coinfection (p<0.005). According to the hemogram results, leukocyte levels were lower in the VL group (p < 0.0001). Additionally, AST (aspartate aminotransferase) and ALT (alanine aminotransferase) levels differed between the groups, with higher levels in patients with VL (p < 0.001). On average, HIV was diagnosed 2.6 years before VL (p < 0.001), and VL relapse was observed only in the coinfection group (36.5% of cases). Fever (β = +0.17; p = 0.032) in the first VL/HIV episode was identified as a risk factor for relapse (R² = 0.18). More deaths occurred in the VL/HIV group (11.1%) than in the VL group (2.4%).

Conclusion/Significance

VL/HIV was found to be prevalent among young adults, although the median patient age was higher in the VL/HIV group. The classic symptomatology of VL was more common in patients not coinfected with HIV; therefore, attention is needed in patients with HIV who present with any symptoms that suggest the presence of VL, especially in endemic areas. No cases of VL relapse occurred in patients without HIV, and death was more common in the VL/HIV co-infected group.

Immunological Paradigms, Mechanisms, and Models: Conceptual Understanding Is a Prerequisite to Effective Modeling

Most mathematical models that describe the individual or collective actions of cells aim at creating faithful representations of limited sets of data in a self-consistent manner. Consistency with relevant physiological rules pertaining to the greater picture is rarely imposed. By themselves, such models have limited predictive or even explanatory value, contrary to standard claims. Here I try to show that a more critical examination of currently held paradigms is necessary and could potentially lead to models that pass the test of time. In considering the evolution of paradigms over the past decades I focus on the “smart surveillance” theory of how T cells can respond differentially, individually and collectively, to both self- and foreign antigens depending on various “contextual” parameters. The overall perspective is that physiological messages to cells are encoded not only in the biochemical connections of signaling molecules to the cellular machinery but also in the magnitude, kinetics, and in the time- and space-contingencies, of sets of stimuli. By rationalizing the feasibility of subthreshold interactions, the “dynamic tuning hypothesis,” a central component of the theory, set the ground for further theoretical and experimental explorations of dynamically regulated immune tolerance, homeostasis and diversity, and of the notion that lymphocytes participate in nonclassical physiological functions. Some of these efforts are reviewed. Another focus of this review is the concomitant regulation of immune activation and homeostasis through the operation of a feedback mechanism controlling the balance between renewal and differentiation of activated cells. Different perspectives on the nature and regulation of chronic immune activation in HIV infection have led to conflicting models of HIV pathogenesis—a major area of research for theoretical immunologists over almost three decades—and can have profound impact on ongoing HIV cure strategies. Altogether, this critical review is intended to constructively influence the outlook of prospective model builders and of interested immunologists on the state of the art and to encourage conceptual work.

Generalized immune activation and innate immune responses in simian immunodeficiency virus infection

PURPOSE OF REVIEW

Chronic immune activation is a key factor driving the immunopathogenesis of AIDS. During pathogenic HIV/simian immunodeficiency virus (SIV) infections, innate and adaptive antiviral immune responses contribute to chronic immune activation. In contrast, nonpathogenic SIV infections of natural hosts such as sooty mangabeys and African green monkeys (AGMs) are characterized by low immune activation despite similarly high viremia. This review focuses on the role of innate immune responses in SIV infection.

RECENT FINDINGS

Several studies have examined the role of innate immune responses to SIV as potential drivers of immune activation. The key result of these studies is that both pathogenic SIV infection of macaques and nonpathogenic SIV infections of natural hosts are associated with strong innate immune responses to the virus, high production of type I interferons by plasmacytoid dendritic cells, and upregulation of interferon-stimulated genes (ISGs). However, SIV-infected sooty mangabeys and AGMs (but not SIV-infected macaques) rapidly downmodulate the interferon response within 4-6 weeks of infection, thus resulting in a state of limited immune activation during chronic infection.

SUMMARY

Studies in nonhuman primates suggest that chronic innate/interferon responses may contribute to AIDS pathogenesis. Further, the ability of natural host species to resolve innate immune responses after infection provides a novel avenue for potential immunotherapy.

Decreases in IL-7 levels during antiretroviral treatment of HIV infection suggest a primary mechanism of receptor-mediated clearance

IL-7 is essential for T-cell homeostasis. Elevated serum IL-7 levels in lymphopenic states, including HIV infection, are thought to be due to increased production by homeostatic feedback, decreased receptor-mediated clearance, or both. The goal of this study was to understand how immune reconstitution through antiretroviral therapy (ART) in HIV(+) patients affects IL-7 serum levels, expression of the IL-7 receptor (CD127), and T-cell cycling. Immunophenotypic analysis of T cells from 29 HIV(-) controls and 43 untreated HIV(+) patients (30 of whom were followed longitudinally for ≤ 24 months on ART) was performed. Restoration of both CD4(+) and CD8(+) T cells was driven by increases in CD127(+) naive and central memory T cells. CD4(+) T-cell subsets were not fully restored after 2 years of ART, whereas serum IL-7 levels normalized by 1 year of ART. Mathematical modeling indicated that changes in serum IL-7 levels could be accounted for by changes in the receptor concentration. These data suggest that T-cell restoration after ART in HIV infection is driven predominantly by CD127(+) cells and that decreases of serum IL-7 can be largely explained by improved CD127-mediated clearance.

The mucosal barrier and immune activation in HIV pathogenesis

PURPOSE OF REVIEW

Significant gastrointestinal pathology occurs in progressive HIV and simian immunodeficiency virus (SIV) infections. This review will examine the relationship between the detrimental events to the gastrointestinal tract during the acute phase of infection and disease progression through the chronic phase and, ultimately, AIDS.

RECENT FINDINGS

Gastrointestinal tract CD4 T cells are dramatically depleted in acutely HIV-infected humans and SIV-infected rhesus macaques, sooty mangabeys, and African green monkeys. In addition HIV infection of humans and SIV-infection of rhesus macaques are characterized by enteropathy and increased intestinal permeability. While SIV-infected rhesus macaques and HIV-infected humans manifest chronic and systemic immune activation and microbial translocation, and progress to chronic infection and AIDS, however, SIV-infected sooty mangabeys and African green monkeys do not.

SUMMARY

Recent studies have increased our understanding of the mechanisms relating structural and immunological damage to the gastrointestinal tract during the acute phase of HIV/SIV infection to immune activation and disease progression in the chronic phase.

Pathogenic mechanisms in simian immunodeficiency virus infection

PURPOSE OF REVIEW

Recent work in pathogenic simian immunodeficiency (SIV) infection of Asian macaques and in natural, nonpathogenic SIV infections of African nonhuman primate species has demonstrated that persistent activation has profound effects on CD4+ memory T-cell proliferation, differentiation and survival. Disease progression in pathogenic infection has been closely linked to these dynamics, reflecting a complex interplay of virus-mediated killing, the effects of systemic activation and host regenerative mechanisms. We review these recent advances.

RECENT FINDINGS

Massive depletion of CD4+ effector-memory T cells invariably occurs during acute CCR5-tropic SIV infection, but is initially stabilized by new production of these cells from spared central memory precursors above the threshold required to maintain clinical immune competence. In pathogenic (but not natural, apathogenic) infections, a persistent state of immune activation, characterized by multiple, recurrent bursts of lymphocyte proliferation, differentiation, migration, death and functional modification of 'resting' cells, is associated with progressive depletion of central memory CD4+ T cells, and ultimately, a collapse of effector site CD4+ memory populations that is closely associated with overt immune deficiency.

SUMMARY

The importance of maintaining the regenerative capacity of the central-memory compartment of CD4+ T cells is increasingly evident. Defining the physiologic and molecular mechanisms responsible for instability of the CD4+ central-memory T cell pool could enable new immunotherapeutic interventions.

HIV-1 induced activation of CD4+ T cells creates new targets for HIV-1 infection in human lymphoid tissue ex vivo

We demonstrate mechanisms by which HIV-1 appears to facilitate its own infection in ex vivo-infected human lymphoid tissue. In this system, HIV-1 readily infects various CD4+ T cells, but productive viral infection was supported predominantly by activated T cells expressing either CD25 or HLA-DR or both (CD25/HLA-DR) but not other activation markers: There was a strong positive correlation (r=0.64, P=.001) between virus production and the number of CD25+/HLA-DR+ T cells. HIV-1 infection of lymphoid tissue was associated with activation of both HIV-1-infected and uninfected (bystanders) T cells. In these tissues, apoptosis was selectively increased in T cells expressing CD25/HLA-DR and p24gag but not in cells expressing either of these markers alone. In the course of HIV-1 infection, there was a significant increase in the number of activated (CD25+/HLA-DR+) T cells both infected and uninfected (bystander). By inducing T cells to express particular markers of activation that create new targets for infection, HIV-1 generates in ex vivo lymphoid tissues a vicious destructive circle of activation and infection. In vivo, such self-perpetuating cycle could contribute to HIV-1 disease.

Expression of human CD226 on T cells and natural killer cells and of soluble CD226 in plasma of HIV-1-infected Chinese patients

Our objective was to detect the expression of CD226 on natural killer (NK) cells and T cells, and to measure the amount of soluble CD226 in the plasma of HIV-infected individuals, in order to evaluate the function of CD226 in HIV infection. Thirty-four untreated HIV-1-infected patients and 26 normal controls were enrolled and three-color flow cytometry was used to detect the expression of CD226 on T lymphocytes and NK cells in whole blood samples taken from the patients and normal controls, and in HIV-1SF33-infected peripheral blood mononuclear cells (PBMCs). An enzymelinked immunosorbent assay (ELISA) was used to detect the level of soluble CD226 in the plasma of HIV-infected patients and normal controls and in the supernatant of HIV-1SF33-infected cells. The level of CD226 expression on CD3+, CD4+, and CD8+ T cells and on CD3- CD16+ NK cells of HIV-infected patients was significantly higher than that of normal controls (p < 0.01). The level of soluble CD226 in the plasma of HIV-infected patients was also significantly higher than that of normal controls (p < 0.01). After stimulation with HIV-1SF33, the level of CD226 expression on CD3+ T cells and CD3- CD16+ NK cells of cultured PBMCs reached peak values at 48 h, which was earlier than in uninfected control cells (72 h). The level of soluble CD226 in the supernatant of HIV- 1SF33-infected cell culture was higher than that of uninfected cells, and the level of soluble CD226 in the supernatant of HIV-1SF33-infected cells reached the peak value at 72 h, which was earlier than in uninfected control cells (96 h) but later than the time of peak CD226 expression on CD3+ T lymphocytes (48 h). We conclude that CD226 may be involved in the immune response to HIV infection and that further experiments are needed to find the function of CD226 in the pathogenesis of HIV infection.

Characterization of peripheral blood lymphocyte subsets in patients with acute Plasmodium falciparum and P. vivax malaria infections at Wonji Sugar Estate, Ethiopia

We investigated the absolute counts of CD4+, CD8+, B, NK, and CD3+ cells and total lymphocytes in patients with acute Plasmodium falciparum and Plasmodium vivax malaria. Three-color flow cytometry was used for enumerating the immune cells. After slide smears were stained with 3% Giemsa stain, parasite species were detected using light microscopy. Data were analyzed using STATA and SPSS software. A total of 204 adults of both sexes (age, >15 years) were included in the study. One hundred fifty-eight were acute malaria patients, of whom 79 (50%) were infected with P. falciparum, 76 (48.1%) were infected with P. vivax, and 3 (1.9%) were infected with both malaria parasites. The remaining 46 subjects were healthy controls. The leukocyte count in P. falciparum patients was lower than that in controls (P=0.015). Absolute counts of CD4+, CD8+, B, and CD3+ cells and total lymphocytes were decreased very significantly during both P. falciparum (P<0.0001) and P. vivax (P<0.0001) infections. However, the NK cell count was an exception in that it was not affected by either P. falciparum or P. vivax malaria. No difference was found in the percentages of CD4, CD8, and CD3 cells in P. falciparum or P. vivax patients compared to controls. In summary, acute malaria infection causes a depletion of lymphocyte populations in the peripheral blood. Thus, special steps should be taken in dealing with malaria patients, including enumeration of peripheral lymphocyte cells for diagnostic purposes and research on peripheral blood to evaluate the immune status of patients.

Pathogenesis of HIV infection: what the virus spares is as important as what it destroys

Upon transmission to a new host, HIV targets CCR5+ CD4+ effector memory T cells, resulting in acute, massive depletion of these cells from mucosal effector sites. This depletion does not initially compromise the regenerative capacity of the immune system because naive and most central memory T cells are spared. Here, we discuss evidence suggesting that frequent activation of these spared cells during the chronic phase of HIV infection supplies mucosal tissues with short-lived CCR5+ CD4+ effector cells that prevent life-threatening infections. This immune activation also facilitates continued viral replication, but infection and killing of target T cells by HIV are selective and the impact on effector-cell lifespan is limited. We propose, however, that persistent activation progressively disrupts the functional organization of the immune system, reducing its regenerative capacity and facilitating viral evolution that leads to loss of the exquisite target cell-sparing selectivity of viral replication, ultimately resulting in AIDS.

Insufficient production and tissue delivery of CD4+ memory T cells in rapidly progressive simian immunodeficiency virus infection

The mechanisms linking human immunodeficiency virus replication to the progressive immunodeficiency of acquired immune deficiency syndrome are controversial, particularly the relative contribution of CD4⁺ T cell destruction. Here, we used the simian immunodeficiency virus (SIV) model to investigate the relationship between systemic CD4⁺ T cell dynamics and rapid disease progression. Of 18 rhesus macaques (RMs) infected with CCR5-tropic SIVmac239 (n = 14) or CXCR4-tropic SIVmac155T3 (n = 4), 4 of the former group manifested end-stage SIV disease by 200 d after infection. In SIVmac155T3 infections, naive CD4⁺ T cells were dramatically depleted, but this population was spared by SIVmac239, even in rapid progressors. In contrast, all SIVmac239-infected RMs demonstrated substantial systemic depletion of CD4⁺ memory T cells by day 28 after infection. Surprisingly, the extent of CD4⁺ memory T cell depletion was not, by itself, a strong predictor of rapid progression. However, in all RMs destined for stable infection, this depletion was countered by a striking increase in production of short-lived CD4⁺ memory T cells, many of which rapidly migrated to tissue. In all rapid progressors (P < 0.0001), production of these cells initiated but failed by day 42 of infection, and tissue delivery of new CD4⁺ memory T cells ceased. Thus, although profound depletion of tissue CD4⁺ memory T cells appeared to be a prerequisite for early pathogenesis, it was the inability to respond to this depletion with sustained production of tissue-homing CD4⁺ memory T cells that best distinguished rapid progressors, suggesting that mechanisms of the CD4⁺ memory T cell generation play a crucial role in maintaining immune homeostasis in stable SIV infection.

T cell dynamics in HIV-1 infection

In the absence of antiretroviral treatment, HIV-1 establishes a chronic, progressive infection of the human immune system that invariably, over the course of years, leads to its destruction and fatal immunodeficiency. Paradoxically, while viral replication is extensive throughout the course of infection, deterioration of conventional measures of immunity is slow, including the characteristic loss of CD4(+) T cells that is thought to play a key role in the development of immunodeficiency. This conundrum suggests that CD4(+) T cell-directed viral cytopathicity alone cannot explain the course of disease. Indeed, recent advances now indicate that HIV-1 pathogenesis is likely to result from a complex interplay between the virus and the immune system, particularly the mechanisms responsible for T cell homeostasis and regeneration. We review these data and present a model of HIV-1 pathogenesis in which the protracted loss of CD4(+) T cells results from early viral destruction of selected memory T cell populations, followed by a combination of profound increases in overall memory T cell turnover, damage to the thymus and other lymphoid tissues, and physiological limitations in peripheral CD4(+) T cell renewal.

HIV, 'an evolving species'. Roles of cellular activation and co-infections

Each small variation of the genome of a species can be preserved if it is useful for the survival of the species in a given environment. Within this framework, the finality of the biological cycle of HIV consists in a search for harmony (biological coherence) with man, which is to say a stable condition. Cellular activation appears to be the strategy developed by HIV in order to achieve this coherence. The price of this strategy is the AIDS. The first contact between HIV and immune system appears to determine the subsequent clinical outcome and the future of HIV. Lymphocytic activation varies during the course of the vital cycle of HIV. For each individual, this lymphocytic activation depends on both the HLA repertoire acquired during thymic ontogenesis and the antigenic experience before and after HIV infection. Thus intercurrent infections alter the immune condition of the organism and influence the outcome of HIV. We described a synthetic analysis of the effects of HIV on the surface protein expression and the cellular activation pathways which should provide insights in the evolutionary relationship between HIV and man and should permit to do a more physiological therapeutic approach.

Modelling the dynamics of LCMV infection in mice: II. Compartmental structure and immunopathology

In this study, we develop a mathematical model for analysis of the compartmental aspects and immunopathology of lymphocytic choriomeningitis virus (LCMV) infection in mice. We used sets of original and published data on systemic (extrasplenic) virus distribution to estimate the parameters of virus growth and elimination for spleen and other anatomical compartments, such as the liver, kidney, thymus and lung as well as transfer rates between blood and the above organs. A mathematical model quantitatively integrating the virus distribution kinetics in the host, the specific cytotoxic T lymphocyte (CTL) response in spleen and the re-circulation of effector CTL between spleen, blood and liver is advanced to describe the CTL-mediated immunopathology (hepatitis) in mice infected with LCMV. For intravenous and "peripheral" routes of infection we examine the severity of the liver disease, as a function of the virus dose and the host's immune status characterized by the numbers of precursor and/or cytolytic effector CTL. The model is used to predict the efficacy of protection against virus persistence and disease in a localized viral infection as a function of the composition of CTL population. The modelling analysis suggests quantitative demands to CTL memory for maximal protection against a wide range of doses of infection with a primarily peripheral site of virus replication without the risk of favoring immunopathology. It specifies objectives for CTL vaccination to ensure virus elimination with minimal immunopathology vs. vaccination for disease.

CD4 T cell depletion is linked directly to immune activation in the pathogenesis of HIV-1 and HIV-2 but only indirectly to the viral load

The causal relationships among CD4 cell depletion, HIV replication, and immune activation are not well understood. HIV-2 infection, "nature's experiment" with inherently attenuated HIV disease, provides additional insights into this issue. We report the finding that in HIV-2 and HIV-1 patients with a comparable degree of CD4 depletion the imbalance in the relative sizes of the naive and memory T cell populations and the up-regulation of CD4 and CD8 cell activation markers (HLA-DR, CD38, CD69, Fas molecules) are similar, even though the viral load in the plasma of HIV-2-infected patients is two orders of magnitude lower than in HIV-1 patients and HIV-2 patients are known to have slower rates of CD4 T cell decline and a better clinical prognosis. Moreover, we found a similar increase in the frequency of cycling CD4 T cells (Ki67+), which was in strong correlation with the expression of activation markers. Finally, the level of T cell anergy, as assessed by the proliferative responses to CD3 stimulation and to a panel of microbial Ags, proved to be comparable in HIV-1 and HIV-2 patients with a similar degree of CD4 depletion despite large differences in viral load. Our data are consistent with a direct causal relationship between immune activation and CD4 cell depletion in HIV disease and an only indirect relation of these parameters to the virus replication rate. Invoking the concept of proximal immune activation and virus transmission, which links efficient transmission of virus to local cell activation and proliferation in response to Ags and inflammation, we propose an integrative interpretation of the data and suggest that strongly elevated immune activation induces CD4 cell depletion and not vice versa, with potential implications for the choice of treatment strategies.

Evidence for increased T cell turnover and decreased thymic output in HIV infection

The effects of HIV infection upon the thymus and peripheral T cell turnover have been implicated in the pathogenesis of AIDS. In this study, we investigated whether decreased thymic output, increased T cell proliferation, or both can occur in HIV infection. We measured peripheral blood levels of TCR rearrangement excision circles (TREC) and parameters of cell proliferation, including Ki67 expression and ex vivo bromodeoxyuridine incorporation in 22 individuals with early untreated HIV disease and in 15 HIV-infected individuals undergoing temporary interruption of therapy. We found an inverse association between increased T cell proliferation with rapid viral recrudescence and a decrease in TREC levels. However, during early HIV infection, we found that CD45RO-CD27high (naive) CD4+ T cell proliferation did not increase, despite a loss of TREC within naive CD4+ T cells. A possible explanation for this is that decreased thymic output occurs in HIV-infected humans. This suggests that the loss of TREC during HIV infection can arise from a combination of increased T cell proliferation and decreased thymic output, and that both mechanisms can contribute to the perturbations in T cell homeostasis that underlie the pathogenesis of AIDS.

Multiple CD4+ cell kinetic patterns and their relationships with baseline factors and virological responses in HIV type 1 patients receiving highly active antiretroviral therapy

This exploratory analyses characterizes patterns of lymphocyte recovery in HIV-1-infected patients treated with highly active antiretroviral therapy (HAART) and investigates their relationship with baseline indices and virologic responses. We modeled kinetics of total CD4+ lymphocytes, as well as naive (CD45RA+ CD62L+), and memory (CD45RA- CD45RO+) subsets in 48 patients treated with AZT/3TC/Ritonavir for 48 weeks in ACTG protocol 315. Cell kinetic indices were estimated by nonlinear regression methods and were correlated with baseline factors and virologic responses. Five different kinetic patterns were identified, including biphasic growth, growth-plateau, growth-depletion, decay-recovery, and biphasic decay. Although overall mean lymphocyte responses showed a biphasic increase in cell number, a careful investigation reveals that only one-third of patients actually followed the biphasic growth pattern in CD4+ cell response, while 44% of 48 patients from this study followed the growth-depletion pattern. CD4+ cell recovery during the first phase and the 48-week study period were negatively correlated with baseline CD4+ cell counts, and positively correlated with baseline viral load. Memory CD4+ cell recovery during the first phase was also negatively correlated with baseline memory CD4+ and total CD4+ cell number, but the recovery rate of memory CD4+ cells during the second phase was positively correlated with baseline CD4+ cell number. Patients with a decay in CD4+ cell count during treatment were more likely to have experienced virological rebound (58%) than patients with nondecay patterns (24%). The rate and magnitude of the absolute increase in total CD4+ and memory CD4+ cell number (but not naive CD4+ cells) during the second phase were lower in patients with viral rebound compared with patients with persistent viral suppression. These results show that the kinetics of lymphocyte reconstitution in response to potent antiretroviral therapy in individual patients vary considerably from the "classic" biphasic increase that characterizes the mean or median response pattern. Pattern analysis of lymphocyte kinetics may be useful for testing relationships among factors that modulate the response to treatment.

The dynamics of CD4+ T-cell depletion in HIV disease

The size and composition of the CD4+ T-cell population is regulated by balanced proliferation of progenitor cells and death of mature progeny. After infection with the human immunodeficiency virus, this homeostasis is often disturbed and CD4+ T cells are instead depleted. Such depletion cannot result simply from accelerated destruction of mature CD4+ T cells - sources of T-cell production must also fail. Ironically, this failure may be precipitated by physiological mechanisms designed to maintain homeostasis in the face of accelerated T-cell loss.

Use of human CD4 transgenic mice for studying immunogenicity of HIV-1 envelope protein gp120

HIV-1 envelope protein, gp120, is a major immunogenic protein of the AIDS virus. A specific feature of this protein is its interaction with the receptor protein, human CD4, an important component of the immune system. This interaction might affect the immunogenic properties of the gp 120 and modulate the immune response towards HIV. To test this hypothesis we used human CD4-transgenic mice for immunization with gp120. The dynamics of the immune response towards gp120, CD4 and other proteins was followed. The results show that the primary immune response to gp120 (two weeks) developed somewhat faster in CD4-transgenic mice versus non-transgenic mice. Both animals, however, ultimately mounted the same level of response over time. The primary immune response to gp120 when complexed with soluble CD4 before the immunization, developed similarly in both groups. The secondary immune response was earlier and markedly stronger in non-transgenic mice compared with the transgenic mice where a less efficient memory response to gp120 was observed. The ability of gp120 to directly interact with CD4+ helper lymphocytes appears to affect the humoral response towards this antigen. Moreover, these effects illustrate how viral modulation of these cells may in turn lead to potentially different states of immunological equilibrium.

T cell depletion in HIV-1 infection: how CD4+ T cells go out of stock

HIV-1 infection is characterized by a gradual loss of CD4+ T cells and progressive immune deficiency that leads to opportunistic infections, otherwise rare malignancies and ultimately death. Extensive research over the past two decades has increased our insight into the pathogenic mechanisms underlying these features of HIV-1 infection. Here, we will give a brief overview of the most recent findings and present a model that fits most of the relevant aspects of HIV-1 infection as known. We hypothesize that HIV-1 infection depletes T cell supplies (which are not replaced because of low and static thymic function) by direct infection and killing of cells and through hyperactivation of the immune system.

Differential dynamics of CD4(+) and CD8(+) T-lymphocyte proliferation and activation in acute simian immunodeficiency virus infection

Although lymphocyte turnover in chronic human immunodeficiency virus and simian immunodeficiency virus (SIV) infection has been extensively studied, there is little information on turnover in acute infection. We carried out a prospective kinetic analysis of lymphocyte proliferation in 13 rhesus macaques inoculated with pathogenic SIV. A short-lived dramatic increase in circulating Ki-67(+) lymphocytes observed at 1 to 4 weeks was temporally related to the onset of SIV replication. A 5- to 10-fold increase in Ki-67(+) CD8(+) T lymphocytes and a 2- to 3-fold increase in Ki-67(+) CD3(-) CD8(+) natural killer cells accounted for >85% of proliferating lymphocytes at peak proliferation. In contrast, there was little change in the percentage of Ki-67(+) CD4(+) T lymphocytes during acute infection, although transient increases in Ki-67(-) and Ki-67(+) CD4(+) T lymphocytes expressing CD69, Fas, and HLA-DR were observed. A two- to fourfold decline in CD4(+) T lymphocytes expressing CD25 and CD69 was seen later in SIV infection. The majority of Ki-67(+) CD8(+) T lymphocytes were phenotypically CD45RA(-) CD49d(hi) Fas(hi) CD25(-) CD69(-) CD28(-) HLA-DR(-) and persisted at levels twofold above baseline 6 months after SIV infection. Increased CD8(+) T-lymphocyte proliferation was associated with cell expansion, paralleled the onset of SIV-specific cytotoxic T-lymphocyte activity, and had an oligoclonal component. Thus, divergent patterns of proliferation and activation are exhibited by CD4(+) and CD8(+) T lymphocytes in early SIV infection and may determine how these cells are differentially affected in AIDS.

Model of HIV-1 disease progression based on virus-induced lymph node homing and homing-induced apoptosis of CD4+ lymphocytes

Several proposed theories have described the progression of HIV infection. Even so, no concrete evidence supports any as comprehensive, including, for example, why the CD4+ T-cell counts fall from 1000/mm3 of blood to roughly 100/mm3 over an average 10-year period, whereas concomitant viral loads are relatively constant, increasing by several orders of magnitude in late-stage disease. Here, we develop and validate a theoretical model that altered lymphocyte circulation patterns between the lymph system and blood due to HIV-induced enhanced lymph-node homing and subsequent apoptosis of resting CD4+ T cells can explain many aspects of HIV-1 disease progression. These results lead to a recalculation of the CD4+ lymphocyte dynamics during highly active antiretroviral therapy, and also suggest new targets for therapy.

Effect of clinical events on plasma HIV-1 RNA levels in persons with CD4+ T-lymphocyte counts of more than 500 x 10(6) cells/l

OBJECTIVE

Immune stimulation of CD4 lymphocytes is thought to enhance HIV-1 replication in vivo. Therefore, we sought to define the impact of clinical events identified as putative immune activators on the variability of plasma HIV-1 RNA levels in persons with CD4 cell counts greater than 500 x 10(6) cells/l.

DESIGN

We prospectively recorded clinical events and measured plasma HIV-1 RNA levels weekly for 24 weeks in 16 HIV-1-infected adults who were not receiving antiretroviral therapy and who had CD4 cell counts greater than 500 x 10(6) cells/l.

METHODS

Standard weekly interviews were conducted to capture potential immune activators (e.g., infections, immunizations, and allergic reactions). All plasma HIV-1 RNA levels were measured using the Amplicor HIV-1 Monitor assay (Roche Diagnostics, Branchburg, New Jersey, USA) according to the manufacturer's instructions.

RESULTS

Participants had remarkably stable viral loads during the 6 month study period. Infections were significantly more frequent during the 7 days prior to individual HIV-1 RNA measurements that exceeded the assay variation thresholds determined for this study (+/- 0.324 log) than during the comparable time periods preceding stable measurements (P = 0.023). As a group, the eight participants who had one to four HIV-1 RNA measurements that exceeded the thresholds experienced more infections and declining CD4 cell counts over the study course compared to the eight participants whose measurements all fell within the thresholds (P = 0.058 and 0.053 respectively).

CONCLUSIONS

Our study suggests that in untreated HIV-1-infected persons with CD4 cell count greater than 500 x 10(6) cells/l, viral load is generally quite stable, although acute minor infections are associated with transient fluctuations generally lasting no more than 1 week.

Population biology of HIV-1 infection: viral and CD4+ T cell demographics and dynamics in lymphatic tissues

Human immunodeficiency virus-1 (HIV-1) is usually transmitted through sexual contact and in the very early stages of infection establishes a persistent infection in lymphatic tissues (LT). Virus is produced and stored at this site in a dynamic process that slowly depletes the immune system of CD4+ T cells, setting the stage for AIDS. In this review, I describe the changes in viral and CD4+ T cell populations in LT over the course of infection and after treatment. I present recent evidence that productively infected CD4+ T cells play an important role in establishing persistent infection from the onset, and that the LT are the major reservoir where virus is produced and stored on follicular dendritic cells (FDCs). I discuss the methods used to define the size of viral and CD4+ T cell populations in LT and the nature of virus-host cell interactions in vivo. These experimental approaches have identified populations of latently and chronically infected cells in which virus can elude host defenses, perpetuate infection, and escape eradication by highly active antiretroviral treatment (HAART). I discuss the dramatic impact of HAART on suppressing virus production, reducing the pool of stored virus, and restoring CD4+ T cell populations. I discuss the contributions of thymopoiesis and other renewal mechanisms, lymphatic homeostasis and trafficking to these changes in CD4+ T cell populations in LT, and conclude with a model of immune depletion and repopulation based on the limited regenerative capacity of the adult and the uncompensated losses of productively infected cells that treatment stems. The prediction of this model is that immune regeneration will be slow, variable, and partial. It is nonetheless encouraging to know that even in late stages of infection, control of active replication of HIV-1 provides an opportunity for the immune system to recover from the injuries inflicted by infection.

Initial increase in blood CD4(+) lymphocytes after HIV antiretroviral therapy reflects redistribution from lymphoid tissues

Previous studies proposed a dynamic, steady-state relationship between HIV-mediated cell killing and T-cell proliferation, whereby highly active antiretroviral therapy (HAART) blocks viral replication and tips the balance toward CD4(+) cell repopulation. In this report, we have analyzed blood and lymph node tissues obtained concurrently from HIV-infected patients before and after initiation of HAART. Activated T cells were significantly more frequent in lymph node tissue compared with blood at both time points. Ten weeks after HAART, the absolute number of lymphocytes per excised lymph node decreased, whereas the number of lymphocytes in the blood tended to increase. The relative proportions of lymphoid subsets were not significantly changed in tissue or blood by HAART. The expression levels of mRNA for several proinflammatory cytokines (IFN-gamma, IL-1beta, IL-6, and macrophage inflammatory protein-1alpha) were lower after HAART. After therapy, the expression of VCAM-1 and ICAM-1 -- adhesion molecules known to mediate lymphocyte sequestration in lymphoid tissue -- was also dramatically reduced. These data provide evidence suggesting that initial increases in blood CD4(+) cell counts on HAART are due to redistribution and that this redistribution is mediated by resolution of the immune activation that had sequestered T cells within lymphoid tissues.

On T-cell dynamics and the hyperactivation theory of AIDS pathogenesis

In an earlier article the author proposed a model of T cell dynamics in which rising activation rates produced falling T-cell counts. Here I show that, in a model of this type, apoptosis and proliferation must nearly balance. Hence, small perturbations of these processes could have large consequences, a fact that may be relevant to AIDS pathogenesis. In addition, I show the model can reproduce all decay rates found by McLean and Michie for cells with radiation-induced chromosome damage, provided that stable damage can proliferate. Memory-cell 'amnesia' is not required. T-cell lifetimes come out much shorter than previously estimated.

HIV infection: how effective is drug combination treatment?

The rate of decline of plasma HIV RNA in patients treated with anti-retroviral drugs has been postulated to reflect the half-lives of previously HIV-infected cells. Here, Zvi Grossman and colleagues argue that the observed decline is explained by the kinetics of ongoing infection cycles. Residual cell-to-cell infection that becomes increasingly difficult to block could stabilize cellular provirus reservoirs.

Multiple modes of cellular activation and virus transmission in HIV infection: a role for chronically and latently infected cells in sustaining viral replication

CD4(+) T cell activation, required for virus replication in these cells, occurs in local microenvironmental domains in transient bursts. Thus, although most HIV originates from short-lived virus-producing cells, it is unlikely that chronic infection is generally sustained in rapid continuous cycles of productive infection as has been proposed. Such continuity of productive infection cycles would depend on efficient long-range transmission of HIV from one set of domains to another, in turn requiring the maintenance of sufficiently high concentrations of cell-free virus across lymphoid tissues at all times. By contrast, long-lived cellular sources of HIV maintain the capacity to infect newly activated cells at close range despite the temporal and spatial discontinuities of activation events. Such proximal activation and transmission (PAT) involving chronically and latently infected cells may be responsible for sustained infection, particularly when viral loads are low. Once CD4 cells are productively infected through PAT, they can infect other activated cells in their immediate vicinity. Such events propagate locally but generally do not spread systemically, unlike in the acute phase of the infection, because of the early establishment of protective anergy. Importantly, antiretroviral drug treatment is likely to differentially impact long-range transmission and PAT.

Increased rates of CD4(+) and CD8(+) T lymphocyte turnover in simian immunodeficiency virus-infected macaques

Defining the rate at which T cells turn over has important implications for our understanding of T lymphocyte homeostasis and AIDS pathogenesis, yet little information on T cell turnover is available. We used the nucleoside analogue bromodeoxyuridine (BrdUrd) in combination with five-color flow cytometric analysis to evaluate T lymphocyte turnover rates in normal and simian immunodeficiency virus (SIV)-infected rhesus macaques. T cells in normal animals turned over at relatively rapid rates, with memory cells turning over more quickly than naive cells. In SIV-infected animals, the labeling and elimination rates of both CD4(+) and CD8(+) BrdUrd-labeled cells were increased by 2- to 3-fold as compared with normal controls. In normal and SIV-infected animals, the rates of CD4(+) T cell BrdUrd-labeling and decay were closely correlated with those of CD8(+) T cells. The elimination rate of BrdUrd-labeled cells was accelerated in both naive and memory T lymphocytes in SIV-infected animals. Our results provide direct evidence for increased rates of both CD4(+) and CD8(+) T cell turnover in AIDS virus infection and have important implications for our understanding of T cell homeostasis and the mechanisms responsible for CD4(+) T cell depletion in AIDS.

Changes in CD4+ and CD8+ T cell subsets in response to highly active antiretroviral therapy in HIV type 1-infected patients with prior protease inhibitor experience

This study explores whether previous failures on antiretroviral drug regimens preclude the possibility of immune restoration. This was assessed by evaluating T cell subset changes in individuals who received a salvage regimen of highly active antiretroviral therapy (HAART) after initially failing protease inhibitor monotherapy. Ten HIV-1-infected asymptomatic patients received a regimen of indinavir, zidovudine, and 3TC after failing saquinavir monotherapy. Changes in absolute numbers of naive, memory, and activated CD4+ and CD8+ T cells expressing a selection of CD45RA, CD62L, CD45RO, HLA-DR, and CD38 markers were monitored prospectively over 6 months. These measurements were correlated with plasma viral load along with alterations in a selected CD8+ V alpha/Vbeta T cell receptor (TCR) repertoire. Over 6 months there was a progressive increase in numbers of CD4+ memory (CD45RA-CD62L+) and naive (CD45RA+CD62L+) T cells, which displayed a modest inverse correlation with viral load. Two phases of CD8+ memory cell changes were identified, consisting of a transient increase in CD45RA+CD62L- numbers after 2 months and thereafter a progressive rise in CD45RA-CD62L+ cells until 6 months. A strong correlation existed between reduced viral load and loss of activated CD8+CD38+HLA-DR+ cell numbers. There was also a temporary broadening of the CD8+ V alpha/Vbeta TCR repertoire at 8 weeks, which became skewed after 6 months in parallel with reduced viral suppression. Closer analysis of naive and memory cell subset proportions in individual patients revealed that enlarged pools of naive subsets were evident in those patients with rebounds in viral load. Overall, drug-experienced patients responding to HAART displayed increased numbers of naive and memory CD4+ subsets, and reduced CD8+ cell activation with a loss of TCR skewing.

Conservation of total T-cell counts during HIV infection: alternative hypotheses and implications

While CD4+ T-cell counts in the blood of HIV-infected individuals gradually decrease, there is a parallel increase in the number of blood CD8+ T cells such that the total number of T cells remains essentially constant for several years (1). The basis and significance of this phenomenon are not known. Based on a statistical analysis of longitudinal T-cell counts from the Transfusion Safety Study (TSS) database and on theoretical considerations, we evaluate several alternative models, including versions of the "blind homeostasis" (BH) hypothesis (1-3). At issue is the nature of the homeostatic regulation of lymphocytes and its apparent failure in HIV infection. The most plausible explanation for the conservation of total blood T-cell numbers while subset ratios change is that CD4+ and CD8+ T cells compete for a limited access to the blood compartment. Such interaction between the subsets implies, in particular, that changes in the number of CD4+ T cells occurring in other tissues cannot be reliably inferred from those observed in the blood. We reiterate propositions made earlier (4) that much of the apparent "depletion" of CD4+ lymphocytes during the asymptomatic phase of HIV infection may be attributed to redistribution between the tissues and the blood compartment.

Direct HIV cytopathicity cannot account for CD4 decline in AIDS in the presence of homeostasis: a worst-case dynamic analysis

The central paradox of HIV pathogenesis is that the viral burden, either free or cellular, seems too low to deplete the CD4 population by direct killing. Until recently, little data could be used to compare direct and indirect pathogenic theories critically. Clinical trials with potent new antiviral agents have measured important kinetic parameters of HIV infection, including viral and infected cell half-lives. This has led to the construction of explicit models of direct killing. Using a worst-case dynamic analysis, we show that such cytopathic models are untenable. Rates of infected cell removal are orders of magnitude too low to suppress steady state CD4 counts significantly in the face of lymphocyte replenishment, especially in early infection. Furthermore, the direct cytopathic models, as proposed, predict an extremely variable disease course across the broad range of observed viral burdens (five orders of magnitude), which is inconsistent with the relatively small differences in disease progression observed between patients. In contrast, immunologic theories of pathogenesis, such as homeostatic dysregulation based on immune activation, do not suffer from these difficulties and are more consistent with the natural history of HIV infection.

Kinetics of CD4+ T cell repopulation of lymphoid tissues after treatment of HIV-1 infection

Potent combinations of antiretroviral drugs diminish the turnover of CD4+ T lymphocytes productively infected with HIV-1 and reduce the large pool of virions deposited in lymphoid tissue (LT). To determine to what extent suppression of viral replication and reduction in viral antigens in LT might lead correspondingly to repopulation of the immune system, we characterized CD4+ T lymphocyte populations in LT in which we previously had quantitated viral load and turnover of infected cells before and after treatment. We directly measured by quantitative image analysis changes in total CD4+ T cell counts, the CD45RA+ subset, and fractions of proliferating or apoptotic CD4+ T cells. Compared with normal controls, we documented decreased numbers of CD4+ T cells and increased proliferation and apoptosis. After treatment, proliferation returned to normal levels, and total CD4+ T and CD45RA+ cells increased. We discuss the effects of HIV-1 on this subset based on the concept that renewal mechanisms in the adult are operating at full capacity before infection and cannot meet the additional demand imposed by the loss of productively infected cells. The slow increases in the CD45RA+ CD4+ T cells are consistent with the optimistic conclusions that (i) renewal mechanisms have not been damaged irreparably even at relatively advanced stages of infection and (ii) CD4+ T cell populations can be partially restored by control of active replication without eradication of HIV-1.