The present study describes the association between inflammatory cell types and feline infectious peritonitis virus (FIPV) antigen in the brain of 4 cats diagnosed as feline infectious peritonitis (FIP). Immunohistochemically, FIPV antigens were detected in the inflammatory foci of the leptomeninges, choroid plexus and ventricles in 3 of the 4 cats. In 3 cases, inflammatory foci mainly consisted of CD204-and Iba1-positive macrophages, and the FIPV antigens were found in the macrophages. In the other case which was negative for FIPV antigen, severe inflammation predominantly consisting of CD20-positive B lymphocytes was observed in the leptomeninges and subventricles, accompanied with diffuse proliferation of gemistocytic astrocytes. The difference in histopathology may reflect the inflammatory process or the strain variation of FIP virus. Feline infectious peritonitis (FIP) is a widely distributed infectious disease caused by feline coronavirus (FCoV) in domestic and wild cat species [1, 2, 11, 13] . FCoV consists of two biotypes including feline enteric coronavirus (FECV) and FIP virus (FIPV). FECV infection is asymptomatic or exclusively causes mild gastrointestinal clinical symptons, while FIPV causes fatal symptoms such as fever, anorexia, weight loss, abdominal distention and neurological symptoms in young cats [2, 13] . The clinical symptoms differ depending on the form and organ distribution of the lesion, and in cases with lesions in central nervous system, neurological symptoms such as depression, ataxia and seizures appear in 85% of cases [15] . It was assumed that FIPV originating from FECV acquired its monocyte-or macrophage-tropism responsible for viral dissemination and developed specific FIP lesions due to vasculitis [17, 18] . The FIP lesions are characterized by localized or disseminated fibrinous, pyogranulomatous, or granulomatous inflammation with protein-rich effusion in the body cavities [9, 10]. These lesions are often accompanied with granulomatous or necrotizing phlebitis and/or periphlebitis, which have been considered as a typical hallmark of FIP [3, 10] . Lesions in the central nervous system (CNS) are frequently observed mainly in the leptomeninges, ventricles, choroid plexus and sometimes neuroparenchyma [5, 12, 16] . Although histopathological features of FIP lesions in the CNS have been well documented [6, 14, 15] , the association between inflammatory cell types and the distribution of viral antigens remains to be clarified. The present study describes the inflammatory cell types and viral antigen distribution in the brain lesions of 4 cats diagnosed as FIP. Also, considering that the FIPV has monocyte and macrophage tropism, immunohistochemical examinations were performed using two kinds of macrophage markers: CD204 (monocyte and macrophage marker) and Iba-1 (microglia and macrophage marker). Four necropsy cases of cats diagnosed as FIP were obtained from the specimen archives of the Laboratory of Veterinary Pathology, Graduate School of Agricultural and Life Sciences, the University of Tokyo. Diagnoses of FIP were mainly based on clinical history, physical examination findings, general blood chemistry and antibody titer for FCoV antigen. Final diagnoses were based on gross findings at necropsy and histopathological findings. Information of the 4 cases, including age, sex, major clinical signs, the antibody titer of FCoV and organs with histologically severe inflammatory lesions is summarized in Table 1. 
Output: NOTE Immunohistochemical studies on meningoencephalitis in feline infectious peritonitis (FIP)