Effects of Alloantigen Exposure and Cell-Associated Mucosal FIV Challenge on Feline Toll-Like Receptor Gene Expression
Department of Veterinary Biosciences, College of Veterinary Medicine, Center for Retrovirus Research, The Ohio State University
Columbus, OH, USA
Toll-like receptors (TLRs) are important players in innate viral immunity. Endosomal TLRs (3,7,8,9) recognize viral nucleic acid while cell surface TLRs (2,4) sense viral protein. Many viruses activate TLRs, including cytomegalovirus, respiratory syncytial virus and HIV. Although often protective, there are circumstances in which TLR activation augments viral pathogenicity. Nine feline TLRs exist and in vitro FIV infection alters TLR expression. Alloantigen exposure also modulates anti-viral immunity. In cats, mucosal exposure to alloantigen induces cellular alloimmune responses that are associated with reduced viral burden after mucosal FIV challenge.
The goal of this study was to investigate TLR gene expression in feline peripheral blood mononuclear cells (PBMC), intra-epithelial (IEL) and lamina propria (LPL) lymphocytes and iliac (ILN), mesenteric (MLN) and popliteal (PLN) lymph nodes to determine if alloantigen exposure and cell-associated mucosal FIV challenge modulate TLR gene expression.
Specific pathogen free (SPF) cats were vaginally exposed weekly for 12 weeks to lymphocyte media (n=7) or allogeneic PBMC in media (n=7). Blood samples were obtained prior to exposure. Twelve weeks post-exposure, 3 cats per group were euthanized to collect blood and tissues. Remaining animals (n= 4 per group) were vaginally challenged with cell-associated FIV. Blood and tissue samples were obtained 12 weeks post-challenge at euthanasia. RNA was extracted and TLR expression was analyzed by real-time RT-PCR. Standard curves were generated from plasmids containing each TLR and GAPDH and TLR copy number was normalized to GAPDH.
Basal TLR expression was variable based upon tissue type. Alloantigen exposure did not alter TLR expression in PBMC, IEL or LPL. However, it did modulate TLR expression in lymph nodes, particularly the MLN where TLR 1,4,5,6 were decreased and TLR 7,9 were increased. TLR expression was also decreased in ILN (TLR9) and PLN (TLR 2,3,8). These findings suggest diffuse nodal alloantigen processing which impacts innate immunity via TLR signaling pathways.
FIV challenge significantly increased TLR 7 expression, decreased TLR 2 expression and had variable effects on TLR 6,8,9 expression in most tissues. This is notable given the endosomal location of TLR 7, 8 and 9 and the importance of these TLRs in anti-viral immunity. FIV exposure had variable effects on the other TLRs. These viral effects on TLR expression were also modulated by prior exposure to alloantigen.
These data indicate a role for TLRs in both alloantigen processing and host response to FIV. Previous data show allogeneic immune responses increase the threshold for susceptibility to mucosal FIV infection. These data suggest this effect may occur via TLR pathways. Additional studies are indicated to further characterize the role of both TLRs and alloantigen exposure in FIV.