Expression of Ia antigen on perivascular and microglial cells after sublethal and lethal motor neuron injury

The expression of immune-associated (MHC class II) antigen was studied immunohistochemically over several months in the rat facial nucleus after nerve transection and after intraneural injection of toxic ricin. Cells expressing Ia antigen were of a perivascular type and parenchymal ramified microglia. In the first few weeks after nerve lesions we observed a gradual increase in the number of Ia-immunoreactive cells starting with an initial appearance of Ia-positive perivascular cells which were succeeded by increasing numbers of Ia-positive ramified microglia. In long-term animals Ia expression was almost exclusively found in microglia. We propose (a) the existence of a population of immunocompetent perivascular cells normally present in adult rat brain that can be stimulated to express Ia antigen, and (b) the existence of a subpopulation of ramified microglia that arises through transformation of Ia-positive perivascular cells in the adult under pathological conditions.