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The role of secondary lymphatic tissue in
immune deficiency of HIV infection, and aging
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Schacker, Timothy
AIDS: September 2008
University of Minnesota, Minneapolis, USA.
Correspondence to Timothy Schacker, Department of Medicine, University of Minnesota, Minneapolis, MN 55455, USA. E-mail: schacoo8@umn.edu
The underlying etiology of the chronic immune activation in progressive HIV is unknown.
HIV infection therefore directly and indirectly causes damage to many immunological compartments, and the combination of these effects is reflected by the immunosuppression and disease progression that accompany HIV infection. A greater understanding of the processes that occur in the secondary lymphoid tissue may provide valuable insights into the underlying pathogenesis of a range of disorders associated with HIV infection.
HIV infection is also a disease of immune activation and T-cell activation is considered to be a central mediator of HIV pathogenesis. Persistent immune activation in HIV-1 infection is associated with progression to AIDS, and a high degree of T-cell activation predicts more rapid disease progression [12]. Immune activation is also central to immune reconstitution with HAART, and persistently elevated levels of CD4 cell activation are associated with incomplete CD4 T-cell recovery despite declines in plasma HIV-RNA levels [13].
The speed and degree of CD4 T-cell reconstitution in HIV-infected patients treated with HAART varies considerably between different immunological compartments and is dependent on the time of initiation of HAART. Whereas CD4 T-cell repopulation of the peripheral blood can be substantial with HAART, repopulation of CD4 T cells in the lymph nodes or gastrointestinal tract can be poor [1-3,5-7]. Gut CD4 cells may increase if therapy is started early after acquisition [8,9], but most patients have substantial depletion (50-60%) of gut lamina propria lymphocytes when sampled 1-7 years after the initiation of antiretroviral therapy
The presence of increased fibrosis in lymph nodes from individuals with advanced HIV has been noted for a number of years [16], but the significance of this has only recently been investigated. Analysis of inguinal lymph nodes from HIV-infected individuals showed that, although the follicular area was relatively spared from fibrosis, significant damage was apparent in the paracortical T-cell zone and associated with marked collagen deposition
In conclusion, HIV infection is primarily a disease of secondary lymphatic tissues and there is growing evidence to suggest that the architectural secondary lymphoid tissue, especially in the gastrointestinal tract, plays an important role in T-cell homeostasis during HIV infection. In HIV infection there is a gradual depletion in CD4 T cells in the peripheral blood as the infection progresses which can be reversed, to a greater or lesser extent, by antiretroviral therapy. However, in GALT, there is a more rapid and massive depletion of CD4 T cells, especially effector memory T cells, that is especially prominent in the acute infection. This depletion alone would impose a considerable homeostatic strain on the maintenance of the naive CD4 T-cell population. The inflammation and tissue remodeling that accompany the immune response to HIV infection leads to fibrosis and disruption of the lymphatic tissue architecture, which further affects the ability of the tissue to maintain support homeostasis and an immune response
The process by which increasing fibrosis leads to the loss of tissue function is analogous to the situation in the liver during chronic hepatitis B and C infection, in which ongoing viral replication in the hepatocytes leads to inflammation, collagen deposition, loss of functional liver tissue and eventually cirrhosis. In lymphatic tissue, the disruption and loss of normal architecture as a result of fibrosis leads to a gradual inability of the lymphatic tissue to maintain a population of CD4 T cells.
A number of compounds that affect fibrosis have been shown to be effective in improving lung function in pulmonary fibrosis. These include pirfenidone, a novel compound shown in vitro to inhibit TGF-β, which as we have discussed is a key stimulus of collagen synthesis and has been implicated in the development of lymphoid tissue fibrosis in HIV. Further studies are required to examine whether reducing fibrosis in the lymphatic system with specific antifibrotic agents or anti-inflammatory agents can improve the CD4 T-cell population in HIV infection, and so whether such intervention may be clinically useful in slowing depletion without antiretroviral therapy, or improving reconstitution with it.
Abstract
HIV infection is a disease primarily of lymphatic tissues, in which most viral replication occurs in CD4 T cells. The most common measures of the impact of HIV infection are made by counting CD4 T cells in peripheral blood. Peripheral blood, however, contains only 2% of the total CD4 cell population in the body and these are typically effector memory cells in transit; the vast majority of CD4 cells reside in the secondary lymphoid tissues (e.g. lymph nodes and mucosal lymphatic tissues) and the impact of HIV replication is most profound on the population residing within these compartments. Within organized follicular aggregates in mucosal tissues and the very precise structures of lymph nodes most viral replication occurs in the parafollicular T-cell zone, both in primary infection and throughout the course of the disease, such that by the time the patient presents with symptoms of HIV seroconversion approximately 50% of the population is already depleted. Therefore, if we are to understand the pathophysiology and pathogenesis of HIV and its related complications fully, we need to examine the structure and function of secondary lymphoid tissues before and during HIV infection and before and during HIV treatment. This may provide valuable insights into the underlying pathogenesis of a range of disorders associated with HIV infection, and potentially aid in the development of therapies aimed at emerging complications of long-term HIV infection.
Introduction
Secondary lymphoid organs and tissues consist of the lymph nodes and spleen, and various mucosal-associated lymphoid tissue such as gut-associated lymphoid tissue (GALT) and bronchial-associated lymphoid tissue (BALT), and are highly active immunological sites that play a critical role in antigen presentation to mature lymphocytes. Approximately 98% of the total CD4 T-cell pool in the body is found in the secondary lymphoid tissue, and many subsets of CD4 cells, including those expressing co-receptors important in HIV infection, are preferentially located to this tissue. Despite the fact that the secondary lymphoid tissue is the primary site of HIV infection and replication, peripheral blood is the most commonly studied immune compartment because of its accessibility. If we are to fully understand the underlying pathophysiology of HIV infection and the response to anti-HIV therapy, it is important to bear in mind that what we see in the peripheral blood does not necessarily fully reflect what is happening in the secondary lymphoid tissue.
To gain a better understanding of what happens in secondary lymphoid tissue we studied lymphatic tissue samples (tonsil or inguinal node) from 23 patients on HAART at similar stages of HIV infection. At the time of biopsy, all but two patients had plasma HIV levels of less than 50 copies/ml and most (81%) had peripheral CD4 T-cell counts that had increased compared with pretreatment levels (median increase 171 cells/µl; range 34-121 cells/µl); the majority of these patients would therefore be considered to have responded to therapy [1]. The increase in peripheral CD4 T cells did not necessarily correspond with increases in lymphatic tissue CD4 T cells. Most of the patients continued to have evidence for marked depletion of CD4 cells in lymph nodes, which varied from almost total absence to approximately 75% of normal (Fig. 1), indicating a marked difference in repopulation on HAART, which did not appear to correlate with the CD4 T-cell count in the peripheral blood [1]. Therefore, despite full suppression of plasma viremia and evidence in the peripheral blood of immune reconstitution, most patients continued to have significant and profound depletion of CD4 cells in organs associated with immunological function.
Gut-associated lymphatic tissue
The GALT makes up over half of the total immune system. Within GALT there are two distinct immunological compartments, lymphoid follicles and the lamina propria. The follicles can resemble secondary lymph nodes and form germinal centers (Peyer's patch) or they can be lymphoid aggregates. Lymphoid aggregates and Peyer's patches are the inductive sites of the GALT, and are where the central memory and naive T cells reside. The lamina propria is the effector site, where effector memory cells that have been programmed to respond to specific pathogens are largely found. In uninfected individuals Peyer's patches are usually found in their greatest numbers in the terminal ileum. In HIV-infected individuals, however, there is a marked reduction in Peyer's patches in the terminal ileum [2]. As discussed later in this article, the loss of Peyer's patches is one consequence of the profound loss of gut-associated CD4 T cells that occurs in HIV infection. As such, the gastrointestinal lymphoid tissue is an important area in the pathophysiology of HIV infection.
In-vivo pathogenesis
CD4 T-cell subsets in secondary lymphoid tissue
Several CD4 T-cell subsets can be identified in secondary lymphatic tissue, which can be classified using different markers. Naive T cells are those CD4 T cells that have not yet been exposed to antigen, and circulate between blood and secondary lymphoid tissues. Memory T cells are programed to respond to previously encountered pathogens. The memory T-cell pool comprises two main distinct populations, central memory and effector memory T cells, which differ in their homing and effector functions. Central memory CD4 T cells have limited effector function, but can proliferate and become effector memory T cells if restimulated by antigen; they therefore play a role in long-term protection and in persistent infection. Effector memory CD4 T cells home to peripheral tissues and sites of inflammation to provide immediate protection against pathogens. Within the effector memory T cell subset there are effector T helper type 1 and T helper type 2 cells that are classified by different functional properties based on their unique cytokine profiles. In normal situations, central memory T cells are primarily found in lymphoid tissues, whereas effector memory T cells are enriched in tissues where they might encounter antigen such as the lamina propria of gut, liver, or the lung. Both sets of CD4 T cells have characteristics that are important in HIV infection; for example, effector memory T cells express the CCR5 co-receptor required for entry of HIV into the cell (CCR5+ CD4 T cells), and these cells in the lamina propria of the mucosa are the primary site of viral replication during the initial stages of HIV infection [3].
Effects of HIV infection on CD4 T cells in secondary lymphatic tissue
There is massive depletion of CD4 T cells in the GALT of HIV-infected patients, which is more substantial than in the peripheral blood or lymph nodes at all stages of infection [2]. This depletion most markedly affects CCR5+ CD4 T cells, the effector memory T cell population. This almost complete depletion of CCR5+ CD4 T cells in the gastrointestinal tract is, however, not reflected by a similar depletion in the lymph nodes or peripheral blood of HIV-infected individuals [2]. Of interest, recent data suggest that the massive depletion of CD4 cells in GALT may be be triggered by virus interactions with an intestinal epithelial cell-associated receptor, GPR15/Bob, which can serve as an alternative co-receptor for HIV-1 and SIV [4].
Reconstitution of CD4 T cells in secondary lymphatic tissue following HAART
The speed and degree of CD4 T-cell reconstitution in HIV-infected patients treated with HAART varies considerably between different immunological compartments and is dependent on the time of initiation of HAART. Whereas CD4 T-cell repopulation of the peripheral blood can be substantial with HAART, repopulation of CD4 T cells in the lymph nodes or gastrointestinal tract can be poor [1-3,5-7]. Gut CD4 cells may increase if therapy is started early after acquisition [8,9], but most patients have substantial depletion (50-60%) of gut lamina propria lymphocytes when sampled 1-7 years after the initiation of antiretroviral therapy [10]. Of interest, a recent study has shown that viral suppression is more effective in the GALT during primary HIV infection compared with chronic infection [11]. Furthermore, genes involved in inflammation and cell activation were found to be upregulated in patients who showed limited repopulation of mucosal CD4 T cells, whereas the expression of genes involved in growth and repair was increased in patients with efficient mucosal CD4 T-cell restoration. These findings suggest that incomplete viral suppression and increased immune activation and inflammation may prevent the restoration of CD4 T cells in gastrointestinal lymphoid tissue during HAART [11].
Drivers of activation in HIV disease
HIV infection is also a disease of immune activation and T-cell activation is considered to be a central mediator of HIV pathogenesis. Persistent immune activation in HIV-1 infection is associated with progression to AIDS, and a high degree of T-cell activation predicts more rapid disease progression [12]. Immune activation is also central to immune reconstitution with HAART, and persistently elevated levels of CD4 cell activation are associated with incomplete CD4 T-cell recovery despite declines in plasma HIV-RNA levels [13].
Microbial translocation and HIV
The underlying etiology of the chronic immune activation in progressive HIV is unknown. As discussed, a rapid, very early and pronounced loss of CD4 T cells occurs in the gastrointestinal tract during HIV infection, causing an intestinal immunodeficiency that may result in opportunist mucosal infections [14]. In addition to this, as seen in both HIV and SIV infection, intestinal damage seems to occur independently of secondary infections (HIV/SIV enteropathy) [14]. One possible source of this activation, therefore, may be microbial translocation across the damaged gastrointestinal tract. To investigate this hypothesis, lipopolysaccharide levels in the plasma were measured as a marker of microbial translocation in patients with HIV infection. Levels of circulating lipopolysaccharide were found to be significantly increased in chronically HIV-infected individuals and also in SIV-infected rhesus macaques (P ˛ 0.002) [15]. The levels of lipopolysaccharide correlated with measures of innate and adaptive immune activation, and increased significantly as the HIV infection progressed (Fig. 2). Interestingly, in non-pathogenic SIV infection in the sooty mangabey, microbial translocation did not seem to occur. These results suggest that microbial translocation might be a principal driver of pathogenesis and CD4 T-cell activation-induced depletion.
Fibrosis of the T-cell zone in secondary lymphoid tissue
The presence of increased fibrosis in lymph nodes from individuals with advanced HIV has been noted for a number of years [16], but the significance of this has only recently been investigated. Analysis of inguinal lymph nodes from HIV-infected individuals showed that, although the follicular area was relatively spared from fibrosis, significant damage was apparent in the paracortical T-cell zone and associated with marked collagen deposition [17]. Using a sensitive quantitative image analysis technique, it can be seen that in patients with early HIV, the high endothelial venules have the highest levels of collagen and that the process of fibrosis spreads from here throughout the T-cell zone. Collagen deposition disrupts the organized structure within the T-cell zone, which is essential for the generation of an immune response and the maintenance of CD4 T-cell populations. The percentage area of collagen in the T-cell zone is inversely correlated with the number of CD4 T cells in the lymphatic tissue [17]. There is also a significant correlation between the amount of collagen in the lymphoid tissue and the magnitude of recovery of the peripheral T-cell pool after HAART [18]. This finding may be related to the fact that the reduction in CD4 T cells with increasing collagen deposition is particularly substantial in the naive population [19], and suggests that, along with other established methods of T-cell depletion such as altered thymopoiesis, increased activation and virus-induced cytotoxicity, HIV-related fibrosis of the lymphatic tissue alters survival, growth and trafficking of naive T cells and so affects the ability of the immune system to maintain and repopulate the CD4 T-cell population [19]. The process by which increasing fibrosis leads to the loss of tissue function is analogous to the situation in the liver during chronic hepatitis B and C infection, in which ongoing viral replication in the hepatocytes leads to inflammation, collagen deposition, loss of functional liver tissue and eventually cirrhosis. In lymphatic tissue, the disruption and loss of normal architecture as a result of fibrosis leads to a gradual inability of the lymphatic tissue to maintain a population of CD4 T cells.
Transforming growth factor beta and fibrosis
The underlying etiology of fibrosis in the lymphoid tissue of HIV-infected individuals is certainly multifactorial, but it is probably related to chronic immune activation and the inflammation response that results from this. One mediator that may be of particular relevance in driving fibrosis, at least partly, is transforming growth factor beta (TGF-β), which is implicated in the fibrotic process of a number of systems and diseases, including pulmonary fibrosis [20]. In SIV-infected rhesus macaques, parallel increases in immune activation, TGF-β and collagen deposition occur after inoculation and collagen deposition is rapid and progressive during the earliest stages of the disease [21]. In this model, TGF-β-positive regulatory T cells and collagen deposition in the acute phase were found to co-localize within the T-cell zone of lymph nodes from SIV-infected animals. These results demonstrate both a temporal and spatial relationship between TGF-β and fibrosis (Fig. 3). The response of TGF-β regulatory T cells would normally be to limit immunoreactivity, thereby preventing immunopathological and autoimmune responses during infections. In SIV infection however, when these cells are not normally regulated and persist, they may potentially have a negative effect by damping down the immune response and by causing collagen deposition that disrupts CD4 T-cell homeostasis.
Lymphatic fibrosis: potential for intervention?
A number of compounds that affect fibrosis have been shown to be effective in improving lung function in pulmonary fibrosis. These include pirfenidone, a novel compound shown in vitro to inhibit TGF-β, which as we have discussed is a key stimulus of collagen synthesis and has been implicated in the development of lymphoid tissue fibrosis in HIV. Further studies are required to examine whether reducing fibrosis in the lymphatic system with specific antifibrotic agents or anti-inflammatory agents can improve the CD4 T-cell population in HIV infection, and so whether such intervention may be clinically useful in slowing depletion without antiretroviral therapy, or improving reconstitution with it.
HIV-induced fibrosis, bronchus-associated lymphoid tissue and pulmonary artery hypertension
BALT is characterized as lymphoid tissue in the lung, analogous to GALT. It has been reported that BALT is not present in the normal human lung, but that it can develop under certain pathological conditions (inducible BALT), including chronic respiratory infection, immunodeficiency and autoimmune disease, in which it functions as an inducible secondary lymphoid tissue for respiratory immune responses [22-24]. There are currently no data on BALT in HIV infection; most work on CD4 T cells and HIV activity in the lung has been performed on bronchoalveolar lavage specimens rather than the tissue samples needed to reveal the in-situ immunological component of lung tissue. The similarity or otherwise of GALT and BALT therefore remains to be clarified.
In conclusion, HIV infection is primarily a disease of secondary lymphatic tissues and there is growing evidence to suggest that the architectural secondary lymphoid tissue, especially in the gastrointestinal tract, plays an important role in T-cell homeostasis during HIV infection. In HIV infection there is a gradual depletion in CD4 T cells in the peripheral blood as the infection progresses which can be reversed, to a greater or lesser extent, by antiretroviral therapy. However, in GALT, there is a more rapid and massive depletion of CD4 T cells, especially effector memory T cells, that is especially prominent in the acute infection. This depletion alone would impose a considerable homeostatic strain on the maintenance of the naive CD4 T-cell population. The inflammation and tissue remodeling that accompany the immune response to HIV infection leads to fibrosis and disruption of the lymphatic tissue architecture, which further affects the ability of the tissue to maintain support homeostasis and an immune response. In HIV-infected individuals, the degree of collagen deposition is inversely correlated with a reduction in the number of CD4 T cells, especially naive CD4 T cells. In addition, the amount of collagen in the T-cell zone of lymphoid tissue is predictive of the extent of reconstitution in peripheral blood and secondary lymphoid tissue after HAART. HIV infection therefore directly and indirectly causes damage to many immunological compartments, and the combination of these effects is reflected by the immunosuppression and disease progression that accompany HIV infection. A greater understanding of the processes that occur in the secondary lymphoid tissue may provide valuable insights into the underlying pathogenesis of a range of disorders associated with HIV infection.
Conflicts of interest: None.
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