Divergent kinetics of proliferating T cell subsets in simian immunodeficiency virus (SIV) infection: SIV eliminates the "first responder" CD4+ T cells in primary infection

Abstract

Although increased lymphocyte turnover in chronic human immunodeficiency virus and simian immunodeficiency virus (SIV) infection has been reported in blood, there is little information on cell turnover in tissues, particularly in primary SIV infection. Here we examined the levels of proliferating T cell subsets in mucosal and peripheral lymphoid tissues of adult macaques throughout SIV infection. To specifically label cells in S-phase division, all animals were inoculated with bromodeoxyuridine 24 h prior to sampling. In healthy macaques, the highest levels of proliferating CD4(+) and CD8(+) T cells were in blood and, to a lesser extent, in spleen. Substantial percentages of proliferating cells were also found in intestinal tissues, including the jejunum, ileum, and colon, but very few proliferating cells were detected in lymph nodes (axillary and mesenteric). Moreover, essentially all proliferating T cells in uninfected animals coexpressed CD95 and many coexpressed CCR5 in the tissues examined. Confocal microscopy also demonstrated that proliferating cells were substantial viral target cells for SIV infection and viral replication. After acute SIV infection, percentages of proliferating CD4(+) and CD8(+) T cells were significantly higher in tissues of chronically infected macaques and macaques with AIDS than in those of the controls. Surprisingly, however, we found that proliferating CD4(+) T cells were selectively decreased in very early infection (8 to 10 days postinoculation [dpi]). In contrast, levels of proliferating CD8(+) T cells rapidly increased after SIV infection, peaked by 13 to 21 dpi, and thereafter remained significantly higher than those in the controls. Taken together, these findings suggest that SIV selectively infects and destroys dividing, nonspecific CD4(+) T cells in acute infection, resulting in homeostatic changes and perhaps continuing loss of replication capacity to respond to nonspecific and, later, SIV-specific antigens.

Fig 1

Percentages and distribution of proliferating CD4⁺ and CD8⁺ T cells in tissues of healthy (noninfected) adult rhesus macaques detected by BrdU incorporation (A) and Ki67 staining (B). Note for both CD4⁺ and CD8⁺ T cells more proliferating cells in blood and spleen than in other tissues (P < 0.05 by Mann-Whitney U test), but there were no significant differences between proliferating CD4⁺ and CD8⁺ T cells, and the ratios and proportions of BrdU⁺ T cells are consistent with those of Ki67⁺ cells in all tissues examined. Graphs represent mean percentages of BrdU⁺/Ki67⁺ cells when gating through either CD4⁺ or CD8⁺ lymphocytes ± SEM. Axi LN, axillary lymph node; Mes LN, mesenteric lymph node; Jej, jejunum; LPL, lamina propria lymphocytes.

Fig 2

Phenotyping of proliferating (BrdU⁺) T cells in various tissues of rhesus macaques. (A and B) Flow cytometry plots show that BrdU⁺ T cells mostly coexpress CD95 (memory marker), suggesting that they are memory cells (A), and substantial numbers of BrdU⁺ cells coexpress CCR5 (HIV/SIV coreceptor) (B) in all tissues from healthy rhesus macaques examined.

Fig 3

Comparison of proliferating (BrdU⁺) CD4⁺ T cells and CD8⁺ T cells from various tissues in uninfected and SIV-infected macaques throughout SIV infection. Data represent mean percentages of CD4⁺ or CD8⁺ T cells labeled with BrdU (mean ± SEM). *, P < 0.05; **, P < 0.01.

Fig 4

Distinct changes in proliferating CD4⁺ (top) and CD8⁺ (bottom) T cells in early stages of SIV infection in tissues of rhesus macaques. Data represent mean percentages of BrdU⁺ cells in gated CD4⁺ or CD8⁺ T cells (mean ± SEM).

Fig 5

Phenotyping of SIV-infected T cells in tissues of infected macaques early in the course of infection by confocal microscopy. As indicated by arrows, several SIV-infected cells (SIV mRNA, red) in tissues appearing white or yellow when dually labeled are also BrdU positive. These results suggest that proliferating cells are major early targets for SIV infection.

Fig 6

Changes in plasma viral load (solid line), CD4⁺ T cells (dotted line, left), and proliferating CD4⁺ T cells (dashed line, right) in blood during early SIV infection (P = 0.036, Spearman r = −0.5).