Basic Science Review
This is the first in a series of periodic reviews of the scientific literature that the Basic Science Project will be making available via the TAG website.
- Exploding CD4 T Cell Counts
- STIs in Chronic Infection: Predicting Success
- Trouble at the Nodes?
- Coughing Up CAF
- Striking the Balance
- T Cell Crosstalk: Communication Via CD40
- Memory CD8 T Cells Hang Tough
Exploding CD4 T Cell Counts
The speed of CD4 T cell reconstitution in people on HAART is variable and can be influenced by many factors, some well established (such as age) and others poorly understood. One important contributor to CD4 T cell reconstitution may be the thymus, which pours out naïve T cells during childhood and adolescence but slows production to more of a trickle into middle and old age.
In an attempt to shed light on the factors associated with rapid CD4 T cell reconstitution, researchers from the University of Modena in Italy identified six individuals (from a cohort of 540) that began with low CD4 nadirs but quickly achieved normal or above-normal counts on HAART. Among the notable features of these "CD4 exploders" were high naïve CD4 T cell counts, high levels of CD4 T cells containing TREC (a marker suggestive of recent exit from the thymus) and high levels of the cytokine interleukin-7 (IL-7). In animal models, IL-7 appears to promote thymic production of new naïve T cells and may also increase the naïve T cell numbers by promoting the division of cells that have already left the thymus. This study suggests that IL-7 may have a role in promoting CD4 T cell recovery in individuals on HAART, but the researchers caution that this cytokine can also promote HIV replication in vitro and will require careful evaluation in animal models before it can be considered for use in humans.
One unusual finding contained in this paper is that the volume of thymic tissue (as assessed by computerized tomography or CT scanning) did not correlate with naïve CD4 T cell reconstitution. This contradicts recent data generated by other researchers and further studies will be needed to reconcile these divergent results.
Features of 'CD4-exploders', HIV-positive patients with an optimal immune reconstitution after potent antiretroviral therapy.
Objective: To identify crucial immunological characteristics of a group of patients, defined 'CD4-exploders', who were able to fully reconstitute their immune system after receiving highly active antiretroviral therapy (HAART).
Changes in thymus volume in adult HIV-infected patients under HAART: correlation with the T-cell repopulation.
An important thymus role has been suggested in T-cell repopulation after HAART in adult HIV-1 infected patients. Thymus volume increase after treatment has been described in HIV-1 infected children but not in adult patients. The objective of this work was to evaluate the effect of HAART on the thymic volume of adult HIV-1 infected patients and its relation with the T-cell repopulation. Twenty-one adult patients following 24 weeks under HAART were included in the study. All patients underwent a thoracic computed tomography (CT) evaluation for the measurement of thymic volumes at weeks 0, 12 and 24. Baseline thymus volume showed a significant correlation with the patient's age. Thymic volume significantly increased after 24 weeks of HAART. Besides, a significant correlation between changes in the thymus volume and changes in both total and naive CD4+ cell counts was found. Only patients with increases >100 CD4+ cell counts after treatment significantly increased the thymic volume. These data show the first evidence of an early change in thymic volume of adult HIV-1 infected patients under HAART. This increase was related to the rise of both total and naive CD4+ cell counts suggesting a functional role of thymic volume increase.
STIs in Chronic Infection: Predicting Success
The question of whether STIs can enhance HIV-specific immunity in chronic infection remains highly controversial. Results to date have been conflicting and often hard to interpret given the variety of study designs employed. The highest response rates to date (based on enhanced control of viral load off therapy) have been reported by Jose Gatell's group in Spain, whose trials involve cycles of 1-3 month long interruptions followed by lengthy re-treatment periods (typically at least six months) and conclude with open-ended interruptions in order to evaluate the viral load setpoint.
Gatell recently analyzed data from three such pilot studies to try and uncover markers that predicted a successful outcome (defined as a viral load of <5,000 copies and at least 0.5 logs below baseline). Significant predictors included lower levels of CD4 T cell activation (as measured by CD38 expression), preserved memory CD4 T cells responses (as measured by proliferative responses to the recall antigen tetanus toxoid and HIV's p24 protein) and lower "naïve" CD4 T cell count (as assessed by expression of CD45RA). It may be worth noting that some researchers favor a stricter definition of naïve CD4 T cells based on co-expression of CD45RA and CD62L, and by this criteria naïve CD4 T cell counts were no longer predictive (p=0.5).
Immunological and virological factors at baseline may predict response to structured therapy interruption in early stage chronic HIV-1 infection.
Background: The objective was to analyse which baseline factors could predict a favourable outcome after structured therapy interruption (STI).
Trouble at the Nodes?
Many studies of HAART-induced immune reconstitution have focused on changes in T cell populations in the peripheral blood, due to the difficulty of sampling lymph node tissue. In a recent study from Ashley Haase's group at the University of Minnesota, the integrity of lymph tissue and the degree of T cell repopulation after prolonged HAART was assessed. The data reveals that HIV-induced changes in the normal lymph node structure can persist despite increases in blood CD4 counts.
In an associated paper from the same group, a specific type of lymph node damage is described that appears to influence CD4 T cell recovery in people on HAART. Analysis of collagen deposits as an indicator of inflammatory damage &/content/art1740.html/content/art1740.html#150; a type of scarring – revealed an inverse association between the amount of collagen deposition and the size of the CD4 T cell population in the lymph node (the greater the collagen deposition, the smaller the size of the CD4 T cell population). There was also an inverse association between the degree of deposition and the magnitude of the CD4 T cell count increase that occurred once study participants started HAART. The researchers state that the scarring appears to be the result of an inflammatory immune response directed against the HIV present in the lymph nodes, and suggest that this finding may support the study of immune-suppressive therapies (such as cyclosporine) as potential treatments. A larger study is now underway which will follow individuals over time and assess whether the lymph node damage can heal during prolonged treatment.
Persistent abnormalities in lymphoid tissues of human immunodeficiency virus-infected patients successfully treated with highly active antiretroviral therapy.
Effective highly active antiretroviral therapy (HAART) for human immunodeficiency virus type 1 is associated with virus suppression and immune reconstitution. However, in some patients, this reconstitution is partial or incomplete because CD4(+) cell counts do not increase significantly. This may be due to damage in the microenvironment of lymphoid tissues (LTs), where CD4(+) T cells reside. To test this hypothesis, LT samples were obtained from 23 patients enrolled in a prospective trial that compared 3 different HAART regimens. Analysis of LT architecture and CD4(+) T cells populations revealed abnormalities in 100% of the LT samples, especially in the follicles, with 43% showing absence, 14% showing regression, and 43% showing hyperplasia. CD4(+) T cell populations were abnormal in 16 (89%) of 18 tissue samples, with 7 (39%) of 18 decreased by >50% of normal levels. These data are consistent with the hypothesis that persistent abnormalities in the microenvironment can influence immune reconstitution and document persistent LT abnormalities with HAART not detected by measures of peripheral CD4(+) T cell count.
Collagen deposition in HIV-1 infected lymphatic tissues and T cell homeostasis
Abstract: Lymphatic tissues (LTs) are structurally organized to promote interaction between antigens, chemokines, growth factors, and lymphocytes to generate an immunologic response and maintain normal-sized populations of CD4+ and CD8+ T cells. Inflammation and tissue remodeling that accompany local innate and adaptive immune responses to HIV-1 replication cause damage to the LT architecture. As a result, normal populations of CD4+ and CD8+ T cells cannot be supported and antigen-lymphocyte interactions are impaired. This conclusion is supported herein following LT sampling before and during anti-HIV therapy in persons with acute, chronic, and late-stage HIV-1 infection. Among seven individuals treated with anti-retroviral therapy (ART) and four individuals deferring therapy we found evidence of significant paracortical T cell zone damage associated with deposition of collagen, the extent of which was inversely correlated with both the size of the LT CD4+ T cell population and the change in peripheral CD4+ T cell count with anti-HIV therapy. The HIV-1-associated inflammatory changes and scarring in LT both limit the ability of the tissue to support and reestablish normal-sized populations of CD4+ T cells and suggest a novel mechanism of T cell depletion that may explain the failure of ART to significantly increase CD4+ T cell populations in some HIV-1-infected persons.
Coughing Up CAF
Researchers have long been searching for the mysterious, HIV-inhibiting "CD8 T cell Antiviral Factor" (CAF) that was originally described by Jay Levy from the University of California at San Francisco. Two recent papers describe potential candidates that may account for some of the anti-HIV activities ascribed to CAF. David Ho's group at Aaron Diamond AIDS Research Center fingered antibacterial molecules known as Defensins that appear to be released from the CD8 T cells of long-term non-progressors. However, according to Jon Cohen's accompanying article in Science, many researchers remain to be convinced that CAF and Defensins are connected.
Researchers from Bruce Walker's group in Boston took a different tack, investigating whether CD8 T cells release substances that do not exert direct anti-HIV effects but rather modify existing proteins in a way that produces antiretroviral activity. They show that activated CD8 T cells from HIV-infected individuals can transform serum bovine antithrombin III into an inhibitory factor capable of suppressing the replication of X4-using HIV isolates. However, the lack of inhibition of R5-using strains (which comprise most primary isolates) suggests that additional work is needed before the mystery of CAF's identity can be considered solved.
Contribution of human-defensin-1, -2 and -3 to the anti-HIV-1 activity of CD8 antiviral factor
It is known since 1986 that CD8 T lymphocytes from certain HIV-1-infected individuals who are immunologically stable secrete a soluble factor, termed CAF, that suppresses HIV-1 replication. However, the identity of CAF remained elusive despite an extensive search. By means of a protein-chip technology, we identified a cluster of proteins that were secreted when CD8 T cells from long-term non-progressors with HIV-1 infection were stimulated. These proteins were identified as -defensins-1, -2, and -3 on the basis of specific antibody recognition and amino-acid sequencing. CAF activity was eliminated or neutralized by an antibody specific for human -defensins. Synthetic and purified preparations of -defensins also inhibited the replication of HIV-1 isolates in vitro. Taken together, our results indicate that -defensins-1, -2, and -3 collectively account for the anti-HIV-1 activity of CAF that is not attributable to ß-chemokines.
Purification of a modified form of bovine antithrombin III as a human immunodeficiency virus type 1 (HIV-1) CD8+ T-cell antiviral factor (CAF)
CD8+ T-cells secrete soluble factor(s) capable of inhibiting both R5- and X4-tropic strains of HIV-1. CCR5 chemokine ligands, released from activated CD8+ T-cells, contribute to the antiviral activity of these cells. These CC-chemokines, however, do not account for all CD8+ T-cell antiviral factor(s) (CAF) released from these cells, particularly since the elusive CAF can inhibit the replication of X4 HIV-1 strains that use CXCR4 and not CCR5 as a coreceptor. Here we demonstrate that activated CD8+ T-cells of HIV-1 seropositive individuals modify serum bovine antithrombin III into an HIV-1 inhibitory factor capable of suppressing the replication of X4 HIV-1. These data indicate that antithrombin III might play a role in the progression of HIV-1 disease.
Striking the Balance
Antigen-specific CD4 T cells (T-helper cells) can produce an array of cytokines in response to stimulation, but typically fall into one of two broad categories that researchers categorize as T-helper type-1 (Th1) or T-helper type-2 (Th2) cells. Th1 CD4 cells produce the cytokines interferon-gamma and/or interleukin-2 (IL-2), while Th2 cells mainly produce IL-4. Th1 cells are thought to primarily collaborate with CD8 T cells while Th2 cells work with B cells to enhance antibody production (although these are not necessarily exclusive functions; Th1 cells can drive production of certain antibodies, for example). One theory of HIV disease progression (originally proposed by Gene Shearer from the NIH and Mario Clerici from the University of Milan in Italy) is based on the idea that there is a selective loss of HIV-specific Th1 responses and a shift to less effective Th2 responses.
A recent study by Francis Gotch and colleagues refines this picture by analyzing the cytokine production patterns of HIV-specific CD4 T cells from three groups: long-term non-progressors (LTNPs), progressors and what they term "immunologically discordant" progressors (a little-studied group that show progressive CD4 decline despite a persistently low viral load). They report that LTNPs appear to maintain a relatively balanced response, consisting of a mixed population of HIV-specific CD4 T cells capable of making IL-2, interferon-gamma and IL-4. Progressors lacked CD4 T cells capable of manifesting these responses. Immunogically discordant progressors were a more mysterious group, displaying Th1 CD4 T cell responses to some HIV antigens but not others, and generally lacking HIV-specific CD4 T cells capable of making IL-4 (although one individual developed Th2 responses and lost Th1 responses concomitantly with the onset of clinical symptoms).
A balanced type 1/type 2 response is associated with long-term nonprogressive human immunodeficiency virus type 1 infection.
Previous reports have emphasized the requirements for strong type 1 cell-mediated responses in the control of human immunodeficiency virus type 1 (HIV-1). HIV-1 Gag p24-specific CD4 helper T-lymphocyte (HTL) responses have been shown to inversely correlate with viral burden in HIV-1-infected individuals. In this study, peripheral blood mononuclear cells from 70 individuals with chronic progressive HIV-1 infection (clinical progressors), 10 clinical nonprogressors, and 3 immunologically discordant progressors were assessed for HTL proliferation and type 1/type 2 cytokine production. Clinical progressors lacked functional HIV-1-specific HTLs with proliferative and cytokine-producing capacity. Clinical nonprogressors were found to respond to a wide range of HIV-1 antigens from different clades, producing both type 1 and type 2 cytokines. Immunologically discordant progressors responded strongly to clade B Gag p24 with a type 1 cytokine profile but not to other antigens. Thus, in contrast to clinical nonprogressors, neither progressors nor immunologically discordant progressors secreted interleukin-4 (IL-4) in response to HIV-1 antigens. Both clinical nonprogressors and immunologically discordant progressors responded broadly to B clade Gag p24-overlapping peptides. However, IL-4 production in the nonprogressors was restricted to a limited number of p24 peptides. No HIV-1-specific T-cell responses were seen in 20 seronegative controls. Additionally, we observed a rapid type 1 to type 2 shift in the response of one immunologically discordant progressor upon onset of clinical symptoms. These results suggest that a balanced type 1/type 2 profile correlates with successful long-term control of HIV-1.
T Cell Crosstalk: Communication Via CD40
The role of CD4 T cells in maintaining CD8 T cell responses is increasingly well documented in a number of animal and human models of infection. Evidence to date suggests that virus-specific CD4 T cells are important in the control of CMV, Epstein-Barr virus, hepatitis B & C and HIV in humans for this reason. It had previously been thought that CD4 T cells maintained CD8 T cell responses by sending messages via a middle man in the form of dendritic cells (DC). The working model was that CD4 T cells locked onto DC displaying the particular antigen (e.g. a piece of CMV, for example) and persuaded the dendritic cell to display a signaling molecule called CD40 on its surface. The CD4 T cell then departed, and when a CD8 T cell specific for that same antigen arrived and locked onto the dendritic cell, the CD40 molecule sent a message (basically: keep working!) by locking onto a reciprocal molecule on the CD8 T cell surface called CD40 ligand or CD40L.
A new Science paper by researchers at the Necker Institute in France demonstrates that the dendritic cell may not be required to perform this middle-man function. Instead, CD4 and CD8 T cells are able to communicate directly by expressing CD40 and CD40L molecules on their surface. The researchers suggest that therapies able to deliver signals through CD40 may be able to substitute for CD4 T cells under some circumstances, a finding consistent with in vitro results published by Anthony Fauci's laboratory at the NIH.
A role for CD40 expression on CD8+ T cells in the generation of CD8+ T cell memory.
The delivery of CD4 help to CD8+ T cell responses requires interactions between CD40 and CD40 ligand and is thought to occur through antigen-presenting cell (APC) activation. Here we show that generation of memory CD8+ T cells displaying an enhanced capacity for cell division and cytokine secretion required CD4 help but not CD40 expression by the APCs. Activated CD4+ and CD8+ T cells expressed CD40; and in the absence of this protein, CD8+ T cells were unable to differentiate into memory cells or receive CD4 help. These results suggest that, like B cells, CD8+ T cells receive CD4 help directly through CD40 and that this interaction is fundamental for CD8+ T cell memory generation.
Ostrowski MA, Justement SJ, Ehler L, Mizell SB, Lui S, Mican J, Walker BD, Thomas EK, Seder R, Fauci AS.
Ridge JP, Di Rosa F, Matzinger P.
Memory CD8 T Cells Hang Tough
The initial response to any infection (or fake infection in the form of a vaccine) is mounted by naïve T and B cells, which typically undergo a maturation process resulting in a legacy of long-lived "memory" cells specific for that infectious agent. The memory cells are responsible for protecting against any re-exposure to the infection, or – in the case of pathogens that persist in the body (e.g. CMV) – protecting against re-activation of the infection. Recent research focusing on memory CD8 T cells has documented a number of enhanced talents compared to their naïve counterparts: they can become activated more quickly and release infection-fighting cytokines and chemokines almost instantly (compared to a delayed release by naïve CD8 T cells).
New research by Rafi Ahmed's group at Emory University adds to this list by showing that, after becoming activated, memory CD8 T cells resist death (apoptosis) better than naïve CD8 T cells. At the conclusion of the paper, Ahmed and colleagues suggest that a better understanding the mechanisms of memory CD8 T cell generation and maintenance could provide valuable guidance for optimizing STI (Stuctured Treatment Interruption) protocols in HIV infection:
"Our findings may also be useful in optimizing structured treatment interruption of HIV-infected individuals. This treatment results in increasing HIV load by cessation of anti-retroviral therapy for a brief period. In some patients and in non-human primates this is followed by increases in the number of Ag-specific cells, which decline again after therapy is reinstituted. Although HIV infection is a complex process, with expansion of Ag-specific CD8 T cells being influenced by multiple factors, such as viral load, CD4 numbers, and the time of initial HIV exposure, structured treatment interruption is essentially a regimen involving a cyclical increase and decrease in Ag. General principles derived from studies of memory cells and their response to Ag may be applicable and provide guidelines for optimizing structured treatment interruption or therapeutic vaccination regimens."
Differential sensitivity of naive and memory CD8(+) T cells to apoptosis in vivo.
Apoptosis is a critical regulator of homeostasis in the immune system. In this study we demonstrate that memory CD8(+) T cells are more resistant to apoptosis than naive cells. After whole body irradiation of mice, both naive and memory CD8(+) T cells decreased in number, but the reduction in the number of naive cells was 8-fold greater than that in memory CD8(+) T cells. In addition to examining radiation-induced apoptosis, we analyzed the expansion and contraction of naive and memory CD8(+) T cells in vivo following exposure to Ag. We found that memory CD8(+) T cells not only responded more quickly than naive cells after viral infection, but that secondary effector cells generated from memory cells underwent much less contraction compared with primary effectors generated from naive cells (3- to 5-fold vs 10- to 20-fold decrease). Increased numbers of secondary memory cells were observed in both lymphoid and non-lymphoid tissues. When naive and memory cells were transferred into the same animal, secondary effectors underwent less contraction than primary effector cells. These experiments analyzing apoptosis of primary and secondary effectors in the same animal show unequivocally that decreased downsizing of the secondary response reflects an intrinsic property of the memory T cells and is not simply due to environmental effects. These findings have implications for designing prime/boost vaccine strategies and also for optimizing immunotherapeutic regimens for treatment of chronic infections.