IMMUNOPATHOGENESIS OF HUMAN IMMUNODEFICIENCY SYNDROME(HIV) INFECTION

HIV and the Intricate Relationship between Viral Pathogenesis and Immune Defenses

As intracellular parasites, all viruses must be intimately familiar with host cellular machinery and capable of suborning it to support their replication cycle. For HIV, this relationship is particularly complex and intimate because HIV targets, infects, and incapacitates cells central to antimicrobial defenses. Thus, host immune defenses and HIV pathogenesis are inextricably linked. Whereas this parasitic relationship may contribute to the persistence and progression of HIV infection, careful study of the relationship between HIV and the immune system has also yielded important insights into mechanisms of immune homeostasis and host defenses in general. This chapter will examine briefly the proposed mechanisms whereby HIV infects host immune cells, the mechanisms whereby host defenses are mobilized to attenuate HIV replication, the strategies HIV uses to evade host immune responses, and finally, the mechanisms whereby HIV induces immune deficiency that places persons at risk for the opportunistic infections and malignancies that define AIDS.

Acquisition of HIV Infection

In most infectious diseases, a number of factors contribute to the risk of acquisition of infection and to the occurrence of illness after exposure to a pathogenic organism. These include the nature of the exposure (eg, the route, the size of the microbial inoculum), the "virulence" of the microbe, and the nature of the host susceptibility to infection. The nature of the exposure clearly determines the risks of infection. Parenteral exposure to blood infected with HIV carries a substantial risk of infection. Among individuals transfused with blood of HIV-infected persons before screening of blood donors was practiced, the risk of infection approached 100%.(1) Transmucosal infection risks vary according to the site of exposure, with risks of transmission through rectal exposure exceeding the risks of transmission through vaginal exposure and both of the above exceeding the risks of transmission across oral mucosa. Mucosal inflammatory disease tends to enhance the risk of transmission particularly if associated with ulceration. Though firm data are lacking, epidemiologic evidence of seroconversion after accidental needlestick injuries (2) or sexual contact with infected persons with different levels of plasma HIV RNA suggest that the magnitude of the inoculum also contributes to the risk of infection.(3,4) Similarly, mother-to-infant transmission of HIV is enhanced among women with high levels of plasma HIV RNA, even after taking into account other known predictors of transmission,(5-7) and the intensity of exposure to contaminated antihemophilic factor concentrates has been shown to predict the risk of HIV infection among hemophiliacs.(8) Insight gained from persons at high risk for infection yet who persistently remain seronegative indicates that certain genetic loci can dramatically affect risk for acquisition of HIV infection. Specifically, persons homozygous for a 32-base-pair deletion (the so-called delta-32 mutation) in the C-C motif chemokine receptor 5 (CCR5) open reading frame that results in failure of surface expression of this key viral coreceptor are protected from acquisition of HIV infection.(9-11) In the rare instances when such persons have been found to be infected, they appear to acquire infection with viruses that may be capable of entry using the CXC motif chemokine receptor 4 (CXCR4) coreceptor.(12-14) In addition, persons with the -2459G polymorphism in the CCR5 promoter that may result in diminished CCR5 expression also may have a somewhat lower risk of infection than do persons with the alternative -2459A nucleotide at this site.(15) One other rare polymorphism in the CCR5 gene, characterized by a point mutation at position 303, introduces a premature stop codon in the elongating product chain and prevents the expression of a functional CCR5 coreceptor when associated with the delta-32 deletion, also conferring virtually complete resistance to CCR5-using viruses.(16)

As these studies have been performed among groups at risk for infection by both parenteral and mucosal routes, these observations suggest that acquisition of infection is highly dependent upon expression of the HIV coreceptor CCR5. The location of this critical "bottleneck" that requires CCR5 expression remains to be determined. One model proposes that CCR5-receptor availability is critical at the level of the mucosal dendritic (Langerhans) cells, which express CCR5 but much less CXCR4 or other C-type lectin receptors to which HIV may bind to facilitate cellular entry. On the other hand, a nonmucosal location for this bottleneck is suggested by the high prevalence of the CCR5 delta-32 homozygous state among seronegative hemophiliacs who otherwise appear to be at high risk for parenteral acquisition of infection.(17) Importantly however, among HIV seronegative cohorts at very high risk of either parenteral or transmucosal infection, only a minority (eg, 16% in a group of hemophiliacs at >95% risk of infection according to treatment history) are homozygous for the delta-32 mutation,(17) indicating that other mechanisms determine risks for and protection from HIV infection in these settings. Of note, members of several high-risk, HIV-seronegative cohorts have demonstrated immunologic "memory" of HIV exposure. Specifically, mucosal immunoglobulin A (IgA) capable of cross-clade HIV binding and neutralization has been found in genital secretions of some high-risk uninfected persons,(18) and low levels of CD8+ T cells reactive to HIV peptides have been found in circulation in other groups of high-risk seronegative individuals.(19,20) It is not yet clear whether these immune defenses are actually responsible for protection against infection or, alternatively, are a reflection only of exposure while protection is mediated by some other mechanisms that remain to be defined.

Acute Infection

Acute infection with HIV is often associated with a febrile illness and clinical evidence of systemic dissemination of virus to lymphoid tissue, the central nervous system, and other sites. High-level viral replication is reflected in high concentrations of virus in plasma and in lymphoid tissue. Viral replication characteristically peaks and then falls concurrently with the appearance in circulation of virus-specific CD8+ cytotoxic T cells.(21,22) As is the case in numerous other viral infections, these cytotoxic T lymphocytes are able to lyse infected host cells and likely attenuate the magnitude of HIV replication. Although animal models have established the importance of CD8+ cells in control of replication with the related simian immunodeficiency virus (SIV),(23,24) it has been very difficult to establish with certainty the nature of the CD8+ T-cell response that determines optimal control of HIV replication (see "Immune Response to HIV" below). Within several months after acquisition of infection, and in the absence of antiviral therapy, a "steady-state" level of HIV replication is established. This level tends to remain relatively stable for many years in a given individual but can vary enormously from person to person. A number of factors may determine steady-state HIV replication levels and these likely include the nature of host adaptive immune defenses, heterogeneities in viral replicative capacity, and heterogeneities in intrinsic host factors that may affect the magnitude of viral propagation.

Antibodies reactive with HIV antigens appear in circulation within a few weeks of infection but generally are first detectable after viral levels have begun to fall to the steady-state level. Although these antibodies often have strong neutralizing activity against the infecting virus, rapid viral escape from neutralization is characteristic, reflecting the enormous adaptability of the viral envelope, including its ability to revise its glycosylation sites, resulting in altered 3-dimensional configuration sufficient to escape antibody-mediated neutralization.(25)

Viral Reservoirs

Whereas most HIV replication is thought to take place in activated CD4+ T lymphocytes in lymphoid tissue, other cell populations may become infected and may play important roles in the persistence of HIV infection. Resting T cells constitute a significant reservoir of latent HIV that may be activated to complete the replication cycle upon activation of the host cell. At one end of the spectrum, in the activated T cell, multiple cellular factors and the viral Tat protein upregulate HIV transcription, resulting in viral production and ultimately destruction of the host cell.(26) At the other end of the spectrum, fully quiescent T cells, ie, those in the G0 phase of the cell cycle, are incapable of sustaining productive HIV replication, due to blocks in reverse transcription (27) as well as inability to enter the nucleus of the resting cell. Recent evidence indicates that, between those extremes, quiescent cells can be induced by exposure to certain cytokines to move far enough along the cell cycle (ie, to the G1 phase) to remove barriers to reverse transcription. Such cells are therefore susceptible to infection by HIV, but do not undergo full activation and cell cycling.(28) It is thought that these cells subsequently return to the fully quiescent state, in which they are protected from the cytopathic effects of massive viral replication. Infection of quiescent cells thus may establish a repository of infected cells capable of maintaining HIV for many years. How this takes place is not entirely clear but recent studies implicate the role of the HIV Nef protein as inducing a chain of events that renders resting CD4+ T cells susceptible to HIV infection.(29)

It has been shown in vivo that HIV can infect T cells that are not fully activated. During the course of HIV infection, integrated and infection-competent provirus can be found in a population of resting memory CD4+ T cells (30,31) and the frequency of these cells tends to remain stable for years, decreasing only minimally with the administration of combination antiretroviral therapies.(32,33)

Other proposed potential reservoirs of infection include sites within the genitourinary tract (34,35) and certain populations of monocytes and tissue macrophages,(36-38) particularly those in the central nervous system (39) and possibly the kidney.(40) In the resting memory cell compartment, sequence analysis has provided evidence of some replenishment of this reservoir over time. The relative stability and long half-life of these cells indicates that current treatment strategies likely will not be capable of eradication of infection in this compartment.(41) Neither intensive and prolonged administration of antiretroviral therapies (33,41) nor the design of strategies to activate expression of virus from these reservoirs by activating T cells through the T-cell receptor (TCR) or with IL-2 has been able to eradicate virus in infected persons, although the frequency with which virus can be found in resting memory cells has been modestly diminished by these therapies.(42,43) Moreover, mathematical modeling and limited experimental evidence suggest that pharmacologically induced, high-level T-cell stimulation not only is unlikely to eliminate the latent reservoir, but also could potentially lead to T-cell depletion and disrupt CD4/CD8 T-cell homeostasis.(44) Several excellent reviews of the role of cellular reservoirs in the pathogenesis of HIV infection have been published recently.(45-48)

Immune Deficiency and Chronic Infection

Although the precise mechanisms of immune dysfunction remain incompletely understood, virtually every arm of the immune response may be affected by HIV infection.

CD4+ T Cells

Progressive depletion in numbers of circulating CD4+ T cells occurs in almost all cases of untreated HIV infection. The number of circulating CD4+ T cells is widely used as a measure of global "immune competence" and provides a predictor of the immediate risk for opportunistic illnesses.(49) Earlier in the course of infection, many HIV-infected persons have a syndrome of generalized lymphadenopathy characterized by accumulation of lymphocytes within inflammatory lymph nodes and upregulation of adhesion molecule expression. Early in the course of infection, memory CD4+ T cells are selectively depleted from circulation; as disease advances, CD4+ T cells of both the naive and memory phenotype are lost from circulation.(50) In advanced disease, all CD4 cell populations are depleted from circulation and from lymphoid tissue sites.

Functional abnormalities of CD4+ T cells are also characteristic of progressive HIV infection. Failure of CD4+ lymphocytes to undergo cell division, for example, has been demonstrated following stimulation of T cells from infected individuals with antigens or mitogens in vitro. A sequential loss of immune responsiveness to recall antigens, followed by alloantigens and then mitogens has been described.(51) Diminished expression of IL-2 is readily demonstrable (51,52) in cells from HIV-infected individuals and may be related to the proliferation defects. In contrast, expression of interferon-gamma by these cells is often unimpaired,(52) suggesting that the defective responsiveness is not a consequence of depletion of antigen-reactive cells but rather a selective impairment in the ability of these cells to respond after engagement of TCRs. The function of CD4+ T cells that specifically recognize antigens from HIV itself appears to be selectively impaired early in the course of HIV infection (see "Immune Response to HIV" below).

Using anti-TCR antibody stimulation to characterize proliferation defects in CD4+ T cells indicates that proliferation defects in HIV disease are associated with early G1-phase cell-cycle arrest (53) and are more commonly observed in persons who have experienced sustained CD4 cell losses.(54) As a key role of CD4+ T cells is to facilitate immune responses though production of immunomodulatory cytokines, the loss of these cells and the failure of remaining cells to function properly constitutes a critical impairment in immune capability. Specific CD4+ T-cell responses to HIV antigens appear to be selectively and lastingly impaired during early HIV infection (see "Immune Response to HIV" below).

CD8+ T Cells

In early HIV infection, CD8+ T-cell numbers tend to increase, reflecting expansion of memory CD8+ T cells, particularly HIV-reactive cells. CD8 cell expansions persist until far advanced stages of HIV disease, when all T-cell numbers tend to fall.(55) In contrast to memory CD8 cell expansions, proportions of naive CD8 cells tend to fall in early infection, but absolute numbers of these cells do not fall until HIV disease progresses.(50) For example, in earlier disease CD8+ T cells that recognize cytomegalovirus are present in large numbers, but in advanced disease the cytolytic function of CD8+ T cells directed against opportunistic pathogens is demonstrably impaired.(56) It is not entirely clear whether the CD8+ cells present in early disease are functionally "normal," as the maturation phenotype of CD8+ T cells recognizing pathogen-derived peptides has been found to be variably perturbed.(57) Whether this is the cause or the consequence (or the interaction of both) of greater exposure to opportunistic pathogen-derived antigens in HIV-infected immunosuppressed persons is difficult to sort out.

As is seen with CD4+ T cells in HIV infection, CD8+ T cells obtained from HIV-infected persons may fail to proliferate in response to TCR activation in vitro.(58) In this setting, however, it is not clear whether the failure to proliferate is a consequence of failure of CD4+ T-cell help (via provision of IL-2 that is essential for CD8+ T-cell proliferation), a reflection of an intrinsic failure of CD8+ T-cell function, or a consequence of CD8+ T-cell maturation to a predominantly effector phenotype.

B Lymphocytes and Antibody Production

As with cellular immune responses, the humoral immune system in HIV infection is characterized by paradoxical hyperactivation and hyporesponsiveness. Hyperactivation is reflected in dramatic polyclonal hyperglobulinemia, only a portion of which is directed against HIV antigens;(59) bone marrow plasmacytosis;(60) heightened expression of activation molecules on circulating B lymphocytes;(61,62) the presence of autoreactive antibodies in plasma;(59,63) and instances of clinical autoimmunelike disease. B-cell hyperreactivity may contribute to the increased risk of B-cell lymphomas in HIV-infected persons, but no causal link has been clearly established.(64) Neither is the etiology of hyperglobulinemia well understood. Elevated plasma levels of the endogenous B-lymphocyte stimulator have been found in HIV-infected persons (65,66) and this may contribute to the B-lymphocyte activation of HIV infection and AIDS. At the same time, diminished B-lymphocyte responsiveness to antigenic stimulation in vitro is characteristic of HIV-infected persons,(62,67-69) who often fail to develop protective antibody responses after immunization with protein or with polysaccharide vaccines.(70-73) The characterization of antibody responses to polysaccharides as "T-cell independent" is only partially correct. Although antibodies can be induced to polysaccharides in the absence of linked peptides that induce cognate help by proximate CD4+ T cells, these responses are not optimal. Moreover, B-lymphocyte responses to pure sugars still require some degree of T-helper support. Lack of CD4 help may therefore underlie the poor antibody responses to polysaccharides that are seen in HIV infection.

Monocytes and Macrophages

Tissue macrophages are often infected with HIV in vivo (74-77) and, because they are generally not killed by the virus, may serve as reservoirs for viral replication. In tissue sites, infected macrophages may be the source not only of viral proteins but also of inflammatory mediators of pathology such as proinflammatory cytokines, tumor necrosis factor (TNF), interleukin-1 (IL-1), IL-6, and IL-10, as well as chemotactic chemokines. Circulating monocytes on the other hand seldom have been shown to harbor infectious HIV; moreover, in vitro studies of blood monocytes largely have failed to show substantial HIV-induced impairments in key functions such as antigen presentation and differentiation.(78,79) On the other hand, in vitro infection of monocyte-derived macrophages with HIV dramatically impairs the ability of these cells to ingest and kill foreign microbes (80,81) as well as to present antigen to T cells.(82) These findings suggest that impaired function of these infected cells in vivo may well contribute to the overall immune dysfunction of HIV infection.