Cerebrospinal fluid atypical lymphocytes in Japanese encephalitis

Among atypical lymphocytes (AL) examined morphologically and immunohistochemically in the cerebrospinal fluid (CSF) of adult patients with encephalitis, we distinguished a CD4+ 'type I' AL, with a multilobulated nucleus resembling those of the abnormal cells in adult T-cell leukemia (ATL), from a CD8+ 'type II' AL, a large lymphocyte with basophilic cytoplasm and a nucleus containing coarse chromatin. Type I AL were detected in 7 of 8 patients with Japanese encephalitis (JE), but in none of 11 patients with herpes simplex encephalitis (HSE) and none of 19 patients with unspecified acute viral encephalitis. Type II AL were seen frequently in all three groups. The observation of type I AL in CSF strongly suggests JE, which warrants careful follow-up without antiherpetic drugs. In identifying type I AL, which presumably are virally transformed lymphocytes, care must be taken to distinguish them from leukemic involvement by ATL cells, which frequently includes the meninges. Both type I and type II AL also must be differentiated from lymphoma cells.