Localization of the viral antigen of feline immunodeficiency virus in the lymph nodes of cats at the early stage of infection.

Immunohistochemical examinations of localization of feline immunodeficiency virus (FIV) Gag protein were performed on lymph nodes of cats experimentally inoculated with three different strains of FIV (infectious molecular clone of TM 1, Petaluma, and KYO-1 strains), using rabbit anti-FIV Gag serum. The FIV Gag antigens were observed in many follicular dendritic cells (FDCs) and sparsely in small lymphocytes of paracortical area in the lymph nodes of cats inoculated with Petaluma and KYO-1 strains. However, the antigens were present only in small lymphocytes, and not in FDCs of a cat inoculated with infectious molecular clone of the TM1 strain. The cell type differences in expression of the viral antigen in vivo might reflect on the cell tropisms of the FIV strains in vitro. By double immunohistochemical staining with rabbit anti-FIV Gag serum and monoclonal antibodies which recognize feline CD4, feline CD8 or feline pan-T molecules, the FIV Gag-positive lymphocytes were characterized as feline CD4-positive T cells. Since the distributions of FIV Gag antigens were mainly in the FDCs, the FDCs may play an important role as a major reservoir and may be a primary target of FIV at early stages of infection.