Major histocompatibility complex (MHC2+) perivascular macrophages in the axotomized facial motor nucleus are regulated by receptors for interferon-gamma (IFNgamma) and tumor necrosis factor (TNF).

The major histocompatibility complex (MHC) glycoproteins, MHC1 and MHC2, play a key role in the presentation of antigen and the development of the immune response. In the current study we examined the regulation of the MHC2 in the mouse brain after facial axotomy. The normal facial motor nucleus showed very few slender and elongated MHC2+ cells. Transection of the facial nerve led to a gradual but strong upregulation in the number of MHC2+ cells, beginning at day 2 and reaching a maximum 14 days after axotomy, correlated with the induction of mRNA for tumor necrosis factor (TNF) alpha, interleukin (IL) 1beta and interferon-gamma (IFNgamma) and a peak in neuronal cell death. In almost all cases, MHC2 immunoreactivity was restricted to perivascular macrophages that colocalized with vascular basement membrane laminin and macrophage IBA1-immunoreactivity, with no immunoreactivity on phagocytic microglia, astrocytes or invading T-cells. Heterologous transplantation and systemic injection of endotoxin or IFNgamma did not affect this perivascular MHC2 immunoreactivity, and transgenic deletion of the IL1 receptor type I, or TNF receptor type 1, also had no effect. However, the deletion of IFNgamma receptor subunit 1 caused a significant increase, and that of TNF receptor type 2 a strong reduction in the number of MHC2+ macrophages, pointing to a counter-regulatory role of IFNgamma and TNFalpha in the immune surveillance of the injured nervous system.