Immortalized clones of fibroblastic reticular cells activate virus-specific T cells during virus infection

Fibroblastic reticular cells (FRCs) are lymphoid stromal cells essential to T-cell migration and survival. Although FRCs are targets of multiple viral infections, little is known about their role during infection due to the cells' scarcity and difficulty in isolating in vivo. To initiate studies of interactions among FRCs, viruses, and immune cells, we isolated and immortalized CD45^-gp38⁺CD35^-CD31^-CD44⁺VCAM1⁺ cell lines from C57BL/6 mice designated as immortalized FRC. Using these cloned cell lines, we have established that FRCs express the major histocompatibility complex (MHC) II molecule, a factor necessary for stimulation of CD4⁺ T cells thought to be expressed primarily by antigen-presenting cells, along with other T-cell stimulatory ligands in an IFN-γ-dependent manner. In this environment, lymphocytic choriomeningitis virus (LCMV)-infected iFRCs activated naive LCMV-specific CD4⁺ and CD8⁺ T cells while limiting expansion of effector LCMV-specific T cells. Thus, FRCs effectively presented antigen along with activating signals during viral infection using both MHC I and MHC II molecules, illustrating a previously undescribed interaction with CD4⁺ T cells and indicating a unique role for FRCs.