In chronic, asymptomatic SIV disease, SIV RNA+ cells are concentrated within B cell follicles. As CTL are abundant in extrafollicular regions (EF) of secondary lymphoid tissues, but scarce inside the follicle (F), we hypothesized that in acute infection prior to CTL development or in chronic infection after CD8 depletion, compartmentalization of SIV RNA+ cells would diminish.
Rhesus macaques were infected IV with SIVmac239. On day 7 (acute infection), 6 animals were sacrificed and lymph nodes (LN), spleen, and rectum harvested and snap frozen in OCT. Three animals underwent LN biopsy (day 42), treatment with CD8 depleting antibody (day 60), and necropsy on day 84. In situ hybridization for SIV RNA and immunostaining for Ki67, CD4, and CD20 were performed to determine frequencies of SIV RNA+ cells and target cells within F and EF. Wilcoxon matched-pairs signed rank test was used to determine significance.
In acutely infected animals, frequencies of SIV RNA+ cells did not differ significantly in F vs EF of LN (median F:EF 1.2, p=0.6), but were higher in F vs EF in spleen (median F:EF 3.7, p=0.03) and rectum (median F:EF 17; p=0.03). After adjusting for Ki67+CD4+ cells in each compartment, there was a trend in all tissues for more SIV RNA+ to be in F vs EF (medians F:EF: LN 2.2; p=0.06; spleen 1.7; p=0.12; rectum 2.3, p=0.06). In chronically infected animals prior to CD8 depletion, frequencies of SIV RNA+ cells in LN were 2.8- to 60-fold higher in F vs EF. After controlling for Ki67+CD4+ cells, SIV RNA+ cells were 11- to 40-fold higher in F vs EF. Following documented CD8 depletion of LN, differences in F vs EF largely abated with the LN F:EF ratio ranging from 0.7-1.8. This was primarily due to increases in SIV RNA+ cells in EF (range, 4- to 56-fold more vs pre-depletion LN), while increases in F were smaller (range, 1.2-1.9 fold more vs pre-depletion LN). Ki67+CD4+ cell frequencies were not substantially altered by CD8 depletion. After adjusting for Ki67+CD4+ cells, frequencies of SIV RNA+ cells were consistently higher in F vs EF of LN post CD8 depletion (range, F:EF 1.4 to 7.2).
In the absence of virus-specific CTL, the concentration of virus-producing cells in B cell follicles is attenuated, but not eliminated. These data support the hypothesis that virus-specific CTL play a key role in virus compartmentalization, and further suggest that T follicular helper cells are inherently more permissive to SIV than other cells.