Follicular T helper CD4+ (Tfh) cells play a critical role in germinal center (GC) formation and B cell maturation. GCs in lymph nodes (LN), particularly within Tfh cells, are sites for preferential SIV infection and replication. Changes in Tfh phenotype and functions in early acute SIV infection may be a major determinant in the development effective antibody-mediated control of SIV infection.
Eighteen rhesus macaques were intravenously infected with SIVmac251 and underwent staggered necropsy during acute and chronic infection. Tfh cells from surface LN (sLN), mesenteric LN (mLN) and spleen were immunophenotyped. We further examined mLN to quantify and localize viral RNA (vRNA) using immunohistochemistry, and performed gene expression and pathway enrichment analyses on sorted Tfh cells from LNs in resting and stimulated conditions.
The frequency of Tfh cells decreased at day 10 post-infection (p.i.) and partially rebounded after 20 days in all tissues. Using principal component analyses we found similar phenotypic profiles in Tfh from mLN and sLN; in contrast, Tfh isolated from the spleen clustered separately after 10 days p.i. Although plasma viremia (pVL) peaked day 10 p.i., vRNA in mLNs was detectable as early as day 5 p.i. within follicles and the T cell zone. While pVL decreased after 20 days, tissue vRNA was increased until 90 days p.i. but was not preferentially found within the follicles during this early period. Very early following infection, transcriptional profiling of Tfh-related genes showed profound modulation of cytokine production and inflammatory pathways. Moreover, we observed a decrease in Tfh responsiveness to stimulation as early as day 5 p.i. This functional ability was partially recovered after 20 days p.i. irrespective of the increasing vRNA found the tissue. tSNE analyses showed independent clustering pre- and post-infection, and Tfh cells from day 90 p.i. had the most similar profile to pre-infection suggesting a partial recovery in responsiveness in later stages of infection.
SIV infection has a profound effect in Tfh frequencies, phenotypic and genetic profiles across tissues since acute infection. This effect suggests a temporal decrease in Tfh ability to provide B cell help during early stages of infection associated with high levels of viremia in blood and tissues, that may directly impact or delay the early induction of SIV-specific antibody production.