Autoimmune arthritis induces paired immunoglobulin-like receptor B expression on CD4⁺ T cells from SKG mice

The chronic, destructive autoimmune arthritis in SKG mice, which closely resembles human rheumatoid arthritis, is the result of self-reactive T cells escaping thymic deletion. Since the inhibitory receptor LIR-1 is up-regulated on auto-reactive T cells in human rheumatoid arthritis, the role of its murine ortholog PIR-B was investigated. Peripheral CD4⁺ T cells from SKG mice were found to frequently express PIR-B, and this population produces more frequently IL-17 upon in vitro stimulation compared to PIR-B^- cells. A much larger fraction of PIR-B⁺ T cells, however, was found to secret no IL-17, but IFN-γ. With regards to the clinical course of the disease, high frequencies of PIR-B⁺ CD4⁺ T cells were found to be associated with a milder course of arthritis, suggesting that the net effect of PIR-B expression is suppression of autoreactive T cells. Our results indicate that overexpression of PIR-B on IL-17-producing SKG CD4⁺ T cells might represent an effective counter-regulatory mechanism against the destructive potential of those cells. More importantly, a major population of PIR-B⁺ T cells in SKG mice appears to play an inhibitory role by way of their IFN-γ production, since high frequencies of those cells ameliorate the disease.