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The decline in CD4/CD8 ratios in lymph nodes (LNs) of SIV macaques and HIV-infected individuals occurs later than that in blood. In a previous study, long-term SIV-infected macaques were delineated into two groups: (1) those whose LNs had normal CD4/CD8 ratios and (2) those whose LNs had low CD4/CD8 ratios. In the present investigation, LNs, spleens, and blood from these groups have been further analyzed to ascertain the cellular and virological events, particularly those involving CD8+ cells, that occur concomitantly with LN CD4% decline. An increase in the percent of CD69-, IL-2R(p75)-, CD45RA1o CD8+ cells was the most constant event observed in lymphoid tissue from mid- to late-stage SIV-infected monkeys. Such cells were sometimes observed in LNs prior to any other immunological or morphological changes. However, decline in LN CD4/CD8 ratios and the associated degeneration of follicular dendritic cells (FDCs) in the germinal centers (GCs) of these nodes were observed only when both CD8+ cell infiltration of GCs and accumulation of viral antigens within the FDC network could be demonstrated. These dramatic changes were also associated with significantly reduced responsiveness to mitogens throughout the lymphoid compartment. In terms of viral burden, immunological and structural collapse of LNs was not always associated with increased viral DNA levels. Despite the CD4+ cell decline in blood during HIV and SIV infections, the immunological and architectural collapse of the lymphoid compartment, which comprises the bulk of the lymphocytes in the body, appears to be a critical event leading to the onset of AIDS. The present findings suggest that increased CD8+ cell activity as well as decrease in CD4+ cell function both contribute to this process.

Simian immunodeficiency virus infection of macaques is a model for human immunodeficiency virus infection of humans. In vivo-titrated stocks of SIV are essential for the utilization of this model for vaccine development. The elicitation of anti-human cell antibodies by some vaccines prepared in human cells and the related protective effects of the vaccine produced in human cells suggest a need for new macaque-derived SIV stocks. Here we describe the titration and characterization of two stocks of SIVmac that were produced in primary rhesus macaque cells. The first virus is SIVmac251, isolated from tissues of macaque 251, and the second is a molecular clone designated as SIVmac239. A 50% rhesus monkey infectious dose (MID50) was titrated for each virus stock by intravenous inoculation. An additional five macaques were inoculated with 10 MID50 of the SIVmac251 stock and were followed for disease outcome. All five monkeys developed antigenemia by 14 days postchallenge. Two of the five monkeys developed strong anti-SIV humoral immunity, whereas three developed little or no humoral immunity. As has been observed previously, the rapidity of disease progression correlated with the lack of a strong antibody response. The three animals with low humoral immunity died within 7 months of challenge, with antigenemia, cachexia, hypoproteinemia, hypoalbuminemia, weight loss, and intractable diarrhea, while maintaining their circulating CD4 numbers. One animal died at 1.5 years of more typical simian AIDS.

Four pigtailed macaques were inoculated with autologous cells expressing low levels of human immunodeficiency virus type 1 (HIV-1). During the first 10 weeks, infectious virus was recovered from peripheral blood mononuclear cells (PBMCs) and lymph nodes from three of the animals. Subsequently, HIV-1 DNA was frequently detected in uncultured PBMCs from all three animals, and virus was isolated from one of them at weeks 38 and 61. The fourth animal, which was rechallenged at week 10 with cell-free virus isolated from one of the others, never became virus isolation positive, but harbored HIV-1 proviral genomes. These virus infections were accompanied by the development of varied HIV-1-specific humoral immune responses. Antibodies to gp160 were first apparent at week 8 in the three initially infected animals and persisted. The animal from whom virus was isolated at late times also developed persisting antibodies to HIV-1 p24 and gp120. Antibodies to gp120 and gp160 became apparent in the rechallenged animal at 1 week following reinoculation, but they waned with time. In vivo passage of the virus was attempted at week 6. One recipient pigtailed macaque and one recipient cynomolgus monkey failed to become detectably infected following transfusion of virus-positive blood and lymph node cells. The long-term presence of HIV-1-specific antibodies and proviral genomes in these animals, and the recovery of infectious virus more than 1 year following inoculation, are indicative of persistent infection, and confirm previous reports that pigtailed macaques are susceptible to HIV-1.

Infection with a variant of simian immunodeficiency virus (SIVsmm/PBj-14) causes death in juvenile pigtailed macaques within 8 days of infection. The primary pathology is localized to the lymphoid tissues of the gut and spleen. Although the virus is present, the lesions are most consistent with acute reactive inflammation. We studied the serum and tissues for evidence of acute cytokine production often associated with acute inflammation. One factor, IL-6, was found to be significantly increased (> 1000-fold) over all other measured cytokines in all the pigtailed macaques who died acutely. Increased levels of IL-6 were found both in the serum and in the inflamed tissues. mRNA for IL-6 was found in the tissues with the highest protein levels of IL-6. The marked increase in IL-6 and IL-6 mRNA correlated with the virus levels in the tissues and serum as determined by viral isolation, immunohistochemistry, and Northern blot analysis. These findings suggest that the underlying pathogenesis of primary tissue damage, necrosis, and death by PBj-14 is the induction of cytokine production. Although the presence of the virus may be critical for the initiation of these events, the intense inflammatory reaction is associated with the cause of death.

Although loss of CD4+ lymphocytes in peripheral blood is a standard criterion for evaluating the course of HIV disease, little is known about changes within lymphoid organs, which contain the bulk (> 50%) of the body's lymphocytes. Because such studies are feasible only by using non-human primates, we have examined lymph nodes (LNs), spleen, and blood from monkeys infected with two isolates of simian immunodeficiency virus (SIV). During both the acute and chronic phases of these infections, characteristic reductions in the blood CD4+ cell levels are not reflected in LN, where the CD4+ pool remains within normal levels. However, when circulating CD4/CD8 ratios have consistently fallen to approximately 0.5, striking decreases in the percentage of CD4 cells (CD4%) and CD4/CD8 ratios in LN occur concomitantly with dramatic increases in viral antigen expression on follicular dendritic cells within LN germinal centers (GCs). The data suggest that loss from the total T cell pool in minimal until the final stages of SIV and HIV disease and that the immunological deterioration of LN is the event that precipitates the increased susceptibility to infections and progression to AIDS.

A variant of simian immunodeficiency virus (SIVSMM/PBj), isolated from a chronically infected pig-tailed macaque has been shown in previous studies to produce acutely fatal disease uniformly in pig-tailed macaques and in some rhesus macaques. The present study extends investigation of SIVSMM/PBj pathogenesis in rhesus and cynomolgus monkeys. Cynomolgus and rhesus macaques were found to be uniformly susceptible to infection, but as previously reported, the rhesus were found to not be uniform in their response during the acute disease. Homogenized tissues from a rhesus that died acutely from SIVSMM/PBj were passaged to 6 rhesus monkeys in an attempt to increase lethality. Five of 6 rhesus monkeys receiving intravenous inoculation of either spleen (10(3) TCID50) or lymph node (10(5) TCID50) homogenate developed acute disease; 4 died (days 8-10), 1 recovered, and one rhesus remained asymptomatic. Three of 3 cynomolgus macaques and 4 of 4 pig-tailed macaques receiving the same inoculum died acutely within 9 days. Clinical disease in macaques that died was characterized by diffuse lymphadenopathy within 5 days of inoculation and severe diarrhea beginning 1 to 3 days before death. Anorexia, lymphopenia (< 1000 cells/mm3), and mild hypoalbuminemia preceded onset of diarrhea by 24 h. Viral p27 was detected in circulation by day 6 postinfection, with all animals dying acutely having detectable serum p27 and no detectable humoral response. Acute lethality was attributed to severe metabolic acidosis (pH < 7.20) which was observed 24-48 h prior to death in the pig-tailed and cynomolgus macaques. Immunohistochemistry revealed numerous SIV antigen-positive lymphocytes and macrophages in the lymph nodes, spleen, gut-associated lymphoid tissues and gastrointestinal lamina propria. Histopathologic lesions included marked to severe hyperplasia of the T-cell-dependent areas in lymphoid tissues and diffuse nonulcerative lymphohistiocytic gastroenteritis. Surviving rhesus developed strong humoral immune responses to the major SIV proteins.