Journal of the Neurological Sciences (1992) 113 (2) : 177-186. http://dx.doi.org/10.1016/0022-510X(92)90245-G Expression of CD45RC and Ia Antigen in the Spinal Cord in Acute Experimental Allergic Encephalomyelitis: An Immunocytochemical and Flow Cytometric Study P.A. McCombe, B.W. Fordyce, J. de Jersey, G. Yoong and M.P. Pender Abstract We performed immunocytochemical studies to analyze the inflammatory infiltrate and major histocompatibility complex class II (Ia) antigen expression in the spinal cord of Lewis rats with acute experimental allergic encephalomyelitis (EAE) induced by inoculation with myelin basic protein and adjuvants. Using antibodies to lymphocyte markers and other monoclonal antibodies we found that during clinical episodes the inflammatory infiltrate was chiefly composed of T lymphocytes and macrophages. The majority of cells in the inflammatory infiltrate were stained by the W3/25 antibody to CD4 and a proportion was stained by OX22 which labels the high molecular weight form of the leucocyte common antigen (CD45RC). CDB+ T cells were sparse and B cells were not detected. There was minimal staining with the OX39 antibody to the interleukin-2 receptor. Presumptive microglia, identified by their dendritic morphology, expressed Ia antigen during the clinical episodes and after recovery. The prominence of Ia antigen expression after recovery could indicate that this la expression was associated with downregulation of the encephalitogenic immune response. We also performed flow cytometry studies on cells extracted from the spinal cord of rats before and during attacks of EAE. With flow cytometry, we found that in established disease a mean of 83(SD, 23)% of CD2+ cells were CD4+, and a mean of 27(SD, 12)% of CD2+ cells were CD45RC+. In rats sampled on the first day of signs, a mean of 43(SD, 22)% of CD2+ cells were CD45RC+. In the cells extracted from the spinal cord of rats with established disease a mean of 47(SD, 32)% of macrophages were CD45RC+. Our study has combined an immunocytochemical assessment of tissue sections with quantitative flow cytometry assessment of cells extracted from the spinal cord of rats with acute EAE. We have shown that the majority of T lymphocytes in the spinal cord are CD45RC − . We have also found prominent Ia expression on dendritic cells in acute EAE and after clinical recovery. Keywords: experimental allergic encephalomyelitis; lymphocyte; microglia, myelin basic protein; immunocytochemistry; autoimmunity; Ia antigen; flow cytometry Introduction Experimental allergic encephalomyelitis (EAE) is an autoimmune demyelinating disease induced by inoculation of susceptible animals with central nervous system myelin (CNS) antigens and adjuvants. The pathology of EAE is dominated by inflammation and demyelination in the central and peripheral nervous systems (Raine 1984; Pender 1987). Immunocytochemical studies of different models of EAE have shown that the inflammatory infiltrate consists of macrophages and T lymphocytes. EAE induced by inoculation with myelin basic protein (MBP-EAE) is useful in the study of the pathogenesis of the EAE because the target antigen is well characterized. In the Lewis rat two previous immunocytochemical studies of actively induced MBPEAE (Hickey et al. 1983, 1985) have shown that the predominant inflammatory cells are CD4+ and express the major histocompatibility complex class II (Ia) antigen. In the rat, the monoclonal antibody OX22 labels an epitope found in exon 5 of CD45RC, a high molecular weight form of the leucocyte common antigen which is expressed on B cells and CD4 + and CD8 + lymphocytes (Spickett et al. 1983; Barclay et al. 1987; Thomas and Lefrancois 1988; Powrie and Mason 1990, 1991). OX22 labelling divides CD4+ T lymphocytes into two populations (Spickett et al.