IMMUNOPATHOGENESIS OF HUMAN IMMUNODEFICIENCY SYNDROME(HIV) INFECTION[final part]

Sequestration in Lymphoid Organs

Much of the immune damage that is seen in HIV infection probably results from viral replication and its consequences in lymphoid tissue. In early stages of HIV infection, generalized lymphadenopathy is commonly recognized.(154,155) In this condition, nodes are filled with lymphocytes, and the representation of CD4+ and CD8+ T lymphocytes in these sites is generally reflective of what is seen in circulation.(156) In untreated HIV infection, lymph nodes show inflammation with heightened expression of cytokines such as interferon-gamma, IL-1, IL-2, and IL-12.(157-160) The inflammatory condition of the lymphoid tissues likely is a consequence of high-level HIV replication at these sites. These inflammatory lymph nodes are also characterized by heightened expression of molecules such as intercellular adhesion molecules and vascular cell adhesion molecules.(161) This "sticky" and inflammatory state likely results in sequestration of circulating lymphocytes in these sites. As disease advances, there is progressive destruction of lymphoid architecture (162) and ultimately lymphoid tissues are, as is the circulation, depleted of lymphocytes.

Heightened Destruction

As noted above, the immune deficiency of HIV infection is characterized by immune activation, with an increased frequency of circulating lymphocytes that have been activated to enter the cell cycle. Interestingly, this heightened entry to the cell cycle is often aborted (at least after in vitro cultivation) as the activated cells tend to die by mechanisms of programmed cell death (109) and also, in some studies, by necrotic cell death.(163,164) This may be true especially for CD4+ T cells, in which studies of telomere length fail to show evidence of sustained successful division (165) despite evidence of heightened cellular proliferation and turnover in vivo (166,167) or ex vitro.(168,169) In contrast, though CD8+ T cells in HIV infection also die after activation,(170) CD8+ T-cell populations in HIV infection tend to have shortening of the average telomere length reflective of multiple rounds of successful cellular replication (at least among the surviving cells).(165) Importantly, for CD4+ T cells, the proportion of cells that are activated, the proportion of cells that incorporate label into DNA, and the proportion of cells that can be demonstrated to die via apoptosis ex vivo far exceed the proportion of cells that are demonstrably infected by HIV; the same can be said for CD8+ T cells, which are rarely infected in vivo. Although antigenic stimulation in response to peptide antigens of HIV itself may account for some portion of the observed activation, the proportion of CD8 cells activated to express activation markers (either CD38 or HLA-DR) far exceeds the proportion of cells recognized to be HIV reactive;(107) the same appears to be true for CD4+ T cells (171) although screening for CD4 cell reactivity has been less comprehensive than has screening for CD8 cell reactivity. Therefore, cellular activation and cell death in HIV infection appear to be entirely determined neither by direct cytopathic effects of the virus nor by immune activation driven by specific peptide recognition. Alternative explanations, such as dysregulated activation of T cells though mechanisms other than T-cell receptor activation, are yet unproven.

Diminished Production

Whereas HIV infection clearly is characterized by heightened cellular destruction and turnover, there is also evidence that immune cellular production may be impaired, at least at certain stages of infection. With advancing stages of HIV infection, there is evidence of cellular hypoproductivity in bone marrow. Pancytopenia is not uncommon in advanced AIDS and bone marrow biopsies often reveal evidence of hypoplasia. Moreover, CD34+ hematopoietic progenitor cells in bone marrow appear susceptible to infection with HIV (172) and impairment of the function of these cells has been described.(173,174) The mechanisms whereby bone marrow productivity is impaired in HIV disease are incompletely understood and it is likely that concurrent infection with opportunistic pathogens such as cytomegalovirus and Mycobacterium avium complex may contribute to this suppression in some persons. Nonetheless, with administration of suppressive antiretroviral therapies, peripheral blood cytopenias characteristically improve.(175)

T lymphocytes undergo maturation and rearrangement of T-cell receptor genes in the thymus, where T cells with receptors of very high avidity for host HLAs that bind endogenous peptides are deleted (this prevents too much autoimmune reactivity) as are T cells with very low avidity for the host HLAs (this assures that remaining T cells are potentially capable of recognizing host HLAs that bind foreign peptides). The population of antigen-naive T lymphocytes that emerges contains a diverse distribution of T-cell receptors with specificities capable of recognizing a broad array of peptide antigens bound to the host's own cell-surface HLAs. Although thymic activity is greatest during development and childhood, there is evidence of thymic function in adulthood as well.(176,177) The role of the thymus in HIV disease is complex. Thymus size is often preserved in HIV-infected adults (particularly in older persons) (178,179) and there is indication that thymic output often is maintained in infected persons.(180) Moreover, preliminary data indicate that in some persons with HIV infection, thymic size is actually diminished after suppression of HIV replication,(181) suggesting that in some HIV-infected persons, thymic function (or thymic size at least) increases during uncontrolled HIV replication, perhaps in order to keep up with the increased demands of HIV-induced cellular turnover, and that this demand falls as HIV replication and immune cell destruction are diminished by antiretroviral therapies. However, there is also evidence that HIV can infect thymic stromal cells and that HIV strains capable of using the CXCR4 coreceptor (X4 strains) can infect thymocytes as well.(182) Moreover, there is clear evidence of thymic failure among persons who fail to increase circulating CD4 cell numbers with antiretroviral therapy-induced suppression of HIV replication.(183)

IL-7 is a cytokine that may be important in thymopoiesis and in promoting naive T-cell expansion.(184-186) Circulating IL-7 levels are often elevated in HIV infection, particularly as CD4 cell counts fall below 100 cells/µL,(187,188) suggesting that increased levels of this cytokine may play an important role in driving T-cell production and homeostasis. Studies of IL-7 administration in SIV-infected macaques and in humans are ongoing and may help to elucidate a possible role for this agent in the treatment of HIV-associated immune deficiency.

Predictors of Immune Deterioration in HIV Infection

The rate of disease progression in untreated HIV infection is highly variable, with some individuals progressing rapidly to experience opportunistic infection and death within months of acquisition of infection and others (ie, long-term nonprogressors) remaining entirely well and maintaining normal CD4 cell counts more than 15 years after infection in the absence of antiretroviral treatment. (Approximately half of persons who acquire HIV infection will develop severe disease--AIDS- -within 10 years if not treated with antiretroviral therapies.) Although long-term nonprogressors represent no more than about 5% of HIV-infected individuals, this variability suggests a need to identify the factors that determine rates of disease progression.

From a clinical perspective, rates of disease progression can be quantified by measuring decreases in circulating CD4 cell numbers over time. This index is highly variable among infected persons. A number of factors that predict the risk of disease progression, measured as rate of CD4 cell decline, progression to opportunistic infection or death, or risk of progression to CD4 cell counts of <200/µL, have been identified. Both viral factors and host factors, and likely their interaction, may predict the risk of HIV disease progression.

The magnitude of HIV replication as reflected in plasma HIV RNA levels is one predictor of the risk for HIV disease progression.(189) The relationship between the extent of HIV replication and disease progression, however, is complex and cannot be conceptualized in terms of a simple linear correlation between plasma HIV RNA level and rate of disease progression across all groups of HIV-infected individuals. For instance, disease progression is seen at significantly lower HIV RNA levels in women than in men.(190) In a limited number of cases, viral heterogeneities may explain differences in rates of disease progression. For example, in a small cohort of individuals who were infected by blood transfusion from a single donor in Australia and subsequently experienced a milder disease course than was expected, the infecting viral isolate was found to have a truncated Nef protein.(191,192) Other studies of HIV-infected persons with divergent disease progression rates have failed to identify plausible sequence or functional differences in the long-term repeat (LTR) and Tat sequences, (193,194) but do not rule out the possibility that heterogeneities in these viral sequences may have an impact on disease course.

Switches in envelope sequences resulting in a phenotype that utilizes the CXCR4 coreceptor are associated with evidence of accelerated HIV disease progression,(195) but the details of how coreceptor use determines disease outcome remain to be established.

The complex interplay between viral fitness and disease progression is potentially significant. Since replicative fitness correlates with plasma HIV RNA levels, more "fit" viruses might be expected to produce faster CD4 cell declines, as has been found to be the case in the context of antiviral drug resistance mutations. In the presence of antiviral drug selection pressure, resistance mutations either to nucleoside reverse transcriptase inhibitors (196,197) or to protease inhibitors (198,199) tend to attenuate the CD4 cell decline induced by wild-type virus. Although this effect may be due in part to diminished replicative capacity of these viruses, there is also reason to believe that decreased plasma HIV RNA levels do not completely explain the effect of fitness on CD4 cell levels and that the intrinsically diminished ability of these viruses to cause immunopathology also may play a role.(200,201)

Host genetic factors further determine the magnitude of HIV replication. For example, persons who are heterozygous for the delta-32 base pair deletion in the CCR5 open reading frame have decreased expression of cell-surface CCR5, lower HIV RNA levels, and slower disease progression.(202) Similarly, the G polymorphism in the -2459 sequence of the CCR5 promoter has been associated with decreased plasma HIV RNA levels and a modest decrease in risk of disease progression.(203) Peripheral blood and Langerhans cells obtained from persons with this genotype show diminished levels of HIV replication in vitro.(15,204) The -28G polymorphism in the promoter of the gene for the CCR5 ligand RANTES supports increased expression of RANTES and is associated with a decreased rate of CD4 decline in infected persons.(205) Another chemokine receptor gene polymorphism, the CCR2 64I allele, is also associated with a decreased risk of HIV disease progression.(203) However, as this receptor is not thought to be important in HIV infection, the mechanism for this effect (possible linkage to another genetic polymorphism) remains to be determined. Persons homozygous for a noncoding sequence in the gene for stromal cell-derived factor 1, the natural ligand for CXCR4, were found to have a delayed risk for progression to AIDS (206) but this observation has not been confirmed in other cohorts.(203)

Factors associated with adaptive immune responses to HIV are also indicators of disease progression risk. For example, certain HLA alleles indicate greater or lesser risks of disease progression.(207,208) In addition, viral mutations that impair binding of specific viral peptides to the HLA of a given patient predict higher levels of HIV replication.(209) Thus, the emergence of viral mutations at loci involved in HLA binding permits escape from protective immune defenses. In addition, homozygosity for HLA alleles is associated with a significantly greater risk of disease progression.(210) Because homozygosity (having fewer distinct HLA molecules) effectively decreases the diversity of peptide-HLA combinations available for recognition, potentially protective immune responses to HIV are limited. Finally, even a single amino acid substitution in an HLA molecule that determines which peptides can be bound and presented by this HLA type can determine a differential risk of HIV disease progression.(211) Thus, some degree of individual variation in HIV replication and ultimately in risk for disease progression is determined by HLA type and diversity.

Less is known about the innate immune responses that may limit HIV replication. Innate defenses are responsible for the most rapid responses to microbial invasion; they help to control microbial replication and to activate the more specific adaptive immune responses. A preliminary report suggests that the killer immunoglobulin-like receptor (KIR) allele KIR 3DS1, which activates natural killer cells, in the presence of the HLA-B BW4-80ILE allele, protects against disease progression in HIV infection,(212) suggesting that innate immune function is also important in the control of HIV disease. Additional studies indicate that preservation of the function and numbers of plasmacytoid dendritic cells--the major sources of interferon-alfa--is associated with protection of persons with advanced HIV disease from the occurrence of opportunistic infection.(88,89)

Nonspecific immune activation is a clear consequence of HIV infection and is correlated with increased HIV replication. For example, plasma concentrations of beta-2 microglobulin, TNF and its receptors, neopterin, and the soluble IL-2 receptor CD25 each correlate with magnitude of HIV replication and risk of disease progression.(213) Similarly, the expression of the activation marker CD38 on CD8 cells is correlated with HIV levels in plasma,(105) but in some studies independently adds predictive value for disease progression,(214,215) as does the level of CD38 expression on CD4+ T cells.(216) Moreover, levels of CD38 expression seem to exhibit an inverse linear correlation with the extent of CD4+ T-cell restoration in response to antiretroviral therapy.(217) Data from a cross-sectional study support a model wherein HIV replication drives immune activation that drives CD4 cell losses.(218)

Finally, age at the time of HIV infection also appears to be correlated with risk of disease progression. Data from large cooperative cohort studies indicate that the correlation of age with risk of disease progression and HIV-related mortality persists after adjusting for CD4 cell counts and plasma HIV RNA level.(219) Recent data comparing HIV-infected subjects to age-matched healthy controls suggest that the effect of age on the clinical course of HIV infection may be related to depletion of naive T cells, diminished CD28 expression, and reduced thymic volumes in older individuals.(179)

A Model for the Immunopathogenesis of HIV-Induced Immune Deficiency

As this chapter has shown, the interactions between HIV and the host response are complex and only partially characterized. Nonetheless, the existing data permit us to propose a model for the pathogenesis of immune deficiency in HIV infection. In this model, lymphoid sites of HIV replication serve as the major sites of immunopathology. At these inflammatory sites of viral replication, heightened adhesion molecule expression results in increased trapping of circulating lymphocytes, often reflected clinically as generalized lymphadenopathy. Trapped lymphocytes at these sites are exposed to a number of signals. Some of these signals may be driven by toxic viral products such as free envelope glycoprotein and viral regulatory proteins. Others may be mediated by cytokines induced at these sites during the intensive and sustained exposure to viral antigens or viral replication. Still other signals are driven by T-cell receptor engagement though binding of antigenic peptides. As a result of these events, both CD4 and CD8 T cells are activated in a dysregulated fashion, and activation may be both antigen-driven and antigen-independent (ie, not driven through T-cell receptor engagement). In this model, both antigen- and cytokine- or viral product-mediated T-cell activation induces cells to enter the cell cycle. The outcome of these events is not only heightened immune activation but also heightened cell death, because even physiologic cellular activation generally is accompanied by increased T-cell death.

It is proposed that both physiologic and dysregulated activation contribute to the profound immune activation and accelerated cell death that characterizes HIV infection. In early stages of infection, antigen-driven CD8+ T-cell expansion predominates, possibly because activated CD8+ T-cells are less susceptible than CD4+ T cells to productive and cytopathic HIV infection at lymphoid sites. Nonetheless, sustained exposure to this "toxic" milieu results in surviving CD4 and CD8 T-cell populations that are functionally impaired in their ability to mediate effector functions such as cytolysis, and are also impaired in the ability to expand in response to T-cell receptor engagement. The result is a less effective and less adaptable immune response to opportunistic pathogens. With advanced disease, bone marrow, the thymus, or both may fail to keep up with the heightened demand for cellular production, and severe immunodeficiency ensues. This model suggests that sustained exposure to viral replication results in immunologic impairments that are not readily reversible and that immune suppressive strategies may provide adjunctive value to antiviral therapies, and preliminary data suggest that this may indeed be the case.(54,122,220) Consistent with this, clinical responses to treatment with antiviral therapies are often less effective when treatment is begun at more advanced stages of disease.(221)