Departments of MicrobiologyImmunology and Pathology, University of Texas Medical Branch, Galveston, TX 77555, USA
Correspondence to: Miles W. Lloyd
Abstract
The mechanism by which HIV causes depletion of CD4 lymphocytes remains unknown. Recent studies have demonstrated that HIV binding to resting CD4 lymphocytes causes them to home from the blood into lymph node, and during the homing process, they are induced into apoptosis only to secondary signals through the homing receptors. If this is the principal mechanism of CD4 cell depletion, it can explain many of the events known to occur in HIV-infected individual.
Keywords: AIDS, HIV, homing, lymph nodes, lymphocytes
The problem
HIV-1 disease is characterized by gradual depletion of CD4 T lymphocytes from the blood, which correlates with declining immunocompetency. The mechanism of CD4 cell loss, however, is still not clear. Some proposed mechanisms focus on the CD4 cells that are replicating HIV and include: direct viral-mediated cell lysis (1), cellular or humoral virus-specific immune killing of productively infected cells (2), viral induction of apoptosis upon replication in cells (3), or induction of apoptosis in productively infected cells by certain second signals (TCR, CD4) (4). These mechanisms do not reconcile a major enigma in understanding HIV pathogenesis: the clear data showing that very few cells in vivo are producing virus at any given time (~1 in 10,000 cells) (5,6). Thus it is unlikely that elimination of productively infected cells would account for any significant loss of CD4 lymphocytes.
Other theories on how HIV depletes CD4 lymphocytes focus on the killing of `uninfected' cells because the predominant cells shown to be dying in lymph nodes of HIV patients are `bystander' cells not producing HIV (7). Suggested theories include: `kiss of death' (membrane fusion) of `bystander' cells upon contacting productively infected cells (8), autoimmune killing of uninfected cells (9), HIV superantigen-mediated deletion of certain T cell subpopulations (10) or induction of apoptosis in `bystander' cells by HIV proteins (Tat, gp120) released from productively infected cells (11,12). None of these theories have been substantiated by in vivo data (13,15).
In general, there are three possible ways CD4 T cells could disappear from the blood: (i) the immune system stops making new cells, (ii) the cells die in the blood or (iii) the cells leave the blood. There is evidence to support the first and third possibilities. In adults, renewal of T lymphocytes is slow (requiring ~1 year in bone marrow transplant recipients or after anti-CD4 antibody depletion in vivo) (16,17). If HIV impairs T cell renewal from progenitor cells, then CD8 T cell counts should drop along with CD4, since CD4 and CD8 cells arise from a common CD4/CD8-positive precursor cell. The absolute CD8 cell numbers actually increase above normal levels for most of the disease course (18), while the CD4 lymphocytes drop. However, recent reports show that both naive CD4 and CD8 lymphocytes are reduced, and production of new CD4 cells is retarded in HIV-infected individuals (1922).
Concerning the second possibility above, there are few data to indicate that cells are dying in the blood. Assessment of the frequencies of dying CD4 lymphocytes in the blood has not demonstrated increased numbers over those in uninfected subjects (4,23). However, higher than normal frequencies of dying T cells are found in lymph nodes (24,25) and these are predominantly `bystander cells' (7).
The third possibility of CD4 T cells leaving the blood is also supported by recent studies. HIV binding to resting CD4 T cells (i.e. abortive infection) induces them to home from the blood into lymph nodes (26). HIV-induced homing of abortively infected resting CD4 cells could add an increment of CD4 cell migration from blood to lymph node above that occurring normally. This phenomenon could explain the gradual disappearance of CD4 T cells from the blood. These homing T cells are abortively infected and are not making HIV mRNA (27). Around half of them are induced into apoptosis after entering the lymph node owing to secondary signals through the homing receptors (27). These data can explain the observed `bystander' cell death in lymph nodes at the same time that CD4 cells grossly disappear from blood but not from lymph node.
Mathematical modeling of what is happening to the CD4 lymphocyte population in HIV-positive patients based on changes in the numbers of CD4 cells in the blood has led to the concept that a large number (2x109) are killed and replaced daily (28). This model was based on the premise that the proportional loss of CD4 cells in the blood over time also occurred in the lymph node compartment. Thus, because only 2% of the total lymphocytes are in the blood, the number of lymphocytes disappearing in the blood per day was multiplied by 50. The model needed verification by data, but most studies have not found significantly increased turnover of CD4 lymphocytes in HIV-infected patients (1921). If CD4 cell loss resulted from direct or indirect killing of productively infected cells, the 109 estimated number of dying cells is ~100-fold higher than the number of productively infected cells (~107 total in the body) (6,29). Further, this cell death estimation does not fit with the data showing that 1091010 viruses are produced per day in an infected person (28). That would mean that nearly one cell dies for every single virion produced. Productively infected cells make 2001000 virions per day (30,31). It thus seems very unlikely that as many CD4 cells die per day as virions produced.
Six important features of HIV infection that need to be incorporated in any model of pathogenesis
First, there are three different possible outcomes of HIV infection of a cell: (i) productive infection, which occurs when an activated T cell is infected, resulting in the production of progeny virions; (ii) latent infection, which occurs after viral production shuts down at the end of the productive phase (32); and (iii) abortive infection, which results when resting T cells (comprising >98% of the T cell population) are infected (33). The relative frequencies of the three types of infected cells (10:10:7500 for productive, latent and abortive respectively) in HIV-infected subjects (29) reasonably correspond to the frequencies of activated to resting cell frequencies (1:100). While most models focus on the productively infected cells, the latently and abortively infected populations should also be considered, since they are likely to be important. Second, dying cells are predominantly found in the lymph nodes, not in the blood. Third, as CD4 T cells disappear from the blood, the net number of CD4 cells often increases in lymph nodes and only in late disease do they disappear (24,34). The CD4/CD8 ratio in the blood becomes inverted early and widens throughout the disease, but this does not occur in the lymph node (24,34), where inversion of the CD4/CD8 ratio occurs only late in disease. In fact, the increasing numbers of CD4 T cells in lymph nodes in early disease may contribute to lymphadenopathy, which commonly occurs. Fourth, the dying cells in lymph nodes are `bystander' cells, which are not making HIV RNA. Fifth, one of the earliest immune function defects, which occurs before significant CD4 loss in the blood, is a deficit of Th-recall (memory) activity (35). Sixth, after HAART (highly active anti-retroviral therapy) administration, the partial, relatively rapid rebound of blood CD4 lymphocytes is disproportionally comprised of memory T cells (36). This has been shown to at least partially result from production of new cells (21), especially after 6 months of treatment when new CD4 cells from thymic output can be detected (37). The early rapid rebound may also be due to recompartmentalization of CD4 lymphocytes when HIV-induced homing is stopped.
A proposed mechanism of HIV pathogenesis that explains the six features
Figure 1 illustrates a proposed mechanism of HIV pathogenesis based on data showing that abortive infection of resting CD4 lymphocytes causes them to leave the blood and home into lymph nodes. HIV, produced from the few productively infected cells in lymph nodes, binds to surrounding T cells (9899% of which are resting) and signals them. These signals induce: (i) up-regulation of CD62L (L-selectin) the homing receptor for peripheral lymph nodes, and induces an enhanced ability of these cells to home back to the lymph nodes after they enter the blood (26); and (ii) up-regulation of Fas, a cell surface death molecule (38). Within 12 days, many of these cells will migrate back into the blood as a result of the normal lymphblood circulation process (39), but when they enter the blood, they exhibit accelerated homing back to the lymph node. As the cells home, they transmigrate the high endothelial venules into the lymph nodes, at which point they receive second signals through the homing receptors. This induces approximately half of these cells to die by apoptosis (27). The fact that these homing cells are abortively infected and are not making viral mRNA accounts for the `bystander cell death' phenomenon. It is likely that if a surviving cell encounters an antigen specific for its TCR, it will respond normally and clonally expand (33). The virus then completes its replication cycle and produces viral progeny. This process, in turn, results in a slow, gradual increase in productively infected cells, which causes more resting cells to home and die. In response to HAART, release of infectious virus from the few productively infected cells abates; consequently, little virus can bind to and signal resting CD4 cells. One immediate effect would be that some cells rapidly return to the blood because enhanced homing is stopped. A disproportionate increase in memory T cells would be observed since HIV atypically induces these normally non-homing cells to home.
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Acknowledgments
We are grateful to Drs Faith McLellan, Liqiang Wang, Mary Moslen, Judy Aronson and Robert Shope for reviewing this manuscript and providing helpful suggestions.
Notes
Transmitting editor: L. A. Herzenberg
Received 18 March 1999, accepted 14 June 1999.
References