Experimental depletion of CD8+ cells in acutely SIVagm-infected African Green Monkeys results in increased viral replication

Abstract

Background: In vivo CD8+ cell depletions in pathogenic SIV infections identified a key role for cellular immunity in controlling viral load (VL) and disease progression. However, similar studies gave discordant results in chronically-infected SMs, leading some authors to propose that in natural hosts, SIV replication is independent of cellular immunity. To assess the role of cellular immune responses in the control of SIV replication in natural hosts, we investigated the impact of CD8+ cell depletion during acute SIV infection in AGMs.

Results: Nine AGMs were infected with SIVagm.sab and were followed up to day 225 p.i. Four were intravenously infused with the cM-T807 antibody on days 0 (50 mg/kg), 6, and 13 (10 mg/kg, respectively) post infection (p.i.). CD8+ cells were depleted for up to 28 days p.i. in peripheral blood and LNs in all treated AGMs. Partial CD8+ T cell depletion occurred in the intestine. SIVagm VLs peaked at similar levels in both groups (107-108 RNA copies/ml). However, while VLs were controlled in undepleted AGMs, reaching set-point levels (104-105 RNA copies/ml) by day 28 p.i., high VLs (>106 RNA copies/ml) were maintained by day 21 p.i. in CD8-depleted AGMs. By day 42 p.i., VLs were comparable between the two groups. The levels of immune activation and proliferation remained elevated up to day 72 p.i. in CD8-depleted AGMs and returned to preinfection levels in controls by day 28 p.i. None of the CD8-depleted animals progressed to AIDS.

Conclusion: CD8+ cells are responsible for a partial control of postacute viral replication in SIVagm.sab-infected AGMs. In contrast to macaques, the SIVagm-infected AGMs are able to control viral replication after recovery of the CD8+ T cells and avoid disease progression.

Figure 1

Effect of cM-T807 administration on CD8⁺ T cells of African green monkeys. a. As shown by flow cytometry analysis, the CD8-depleting antibody induced complete depletion in peripheral blood for 14-21 days. Both percentage (upper left panel) and absolute (upper right panel) CD8⁺ T cell counts are shown. In the LN, the depletion, although complete was shorter than in periphery (lower left panel), while in the intestine, only a transient, incomplete depletion was observed (lower right panel). Red symbols and lines denote CD8-depleted monkeys. Black symbols and lines denote the control monkeys. b. Flow-cytometry plots of CD4⁺ and CD8⁺ populations (gated on CD3⁺) in peripheral blood, lymph nodes and intestine demonstrate that administration of CD8-depleting mAb determined depletion in periphery and lymphoid tissue and likely induced down regulation/blocking of CD8 molecule rather than CD8⁺ cell depletion in the intestine. Thus, in monkeys treated with cM-T807 mAb (lower panels), downregulation of CD8 cells in the gut is suggested by the massive increase in the double negative population at the time of maximum depletion (day 8 p.i.) compared to baseline (day 0 p.i.). With the rebound of CD8⁺ cells, this double negative population vanishes, as illustrated here by a plot on samples collected at day 28 p.i.

Figure 2

Immunohostochemistry assessment of the efficacy of CD8 depletion on jejunum. Samples from CD8-depleted AGMs were collected at the baseline, at day 8 p.i. and at day 42.p.i. As illustrated, CD8⁺ cells from both Peyer patches (PP) and lamina propria (LP) are depleted/down regulated.

Figure 3

Dynamics of CD3⁺ cells demonstrated significant decreases in blood (a) and LNs (b), suggestive for CD8⁺ cell depletion at these sites. Conversely, no significant dynamics of the CD3⁺ cells were observed in the intestine (c), thus confirming CD8 down regulation at this site. Black symbols and lines denote the control monkeys. Red symbols and lines denote CD8-depleted monkeys.

Figure 4

Dynamics of SIVagm.sab plasma (a) and PBMC (b) vRNA loads in cM-T807-treated AGMs and control monkeys. In CD8-depleted AGMs there was a delay in the control of acute viral replication. Black symbols and lines denote the control monkeys. Red symbols and lines denote CD8-depleted monkeys.

Figure 5

Changes in CD4⁺ T cells in blood (a), lymph nodes (b) and intestine (c) in CD8-depleted AGMs (red lines and dots) and control monkeys (black lines and dots). The index of mucosal CD4 T cells is calculated as the proportion of CD4⁺ CD3⁺ T cells at different time points relative to the baseline levels. This index illustrates the degree of CD4⁺ T cell depletion.

Figure 6

Dynamics of CD4⁺ and CD8⁺ T cell immune activation (as defined by changes in the expression of MHC class II markers) (a and c) and of CD4⁺ and CD8⁺ T cell proliferation (as defined by changes in the expression of Ki-67) (c and d) in peripheral blood of CD8-depleted AGMs (red lines and dots) and control monkeys (black lines and dots). See text for further detail.

Figure 7

Dynamics of plasma proinflammatory cytokine secretion: 1L-1ra (a), IL-12 (b) and IL-15 (c) in CD8-depleted AGMs (red lines and dots) and control monkeys (black lines and dots). See text for further detail.