942 Measles Virus

A relation has been assumed between the pathogenesis of subacute sclerosing panencephalitis (SSPE) 1 and a preceding infection with measles virus that had been contracted some 10 yr before (1). Although no formal proof exists that measles virus has remained present in individual children suffering from SSPE, this is generally believed to be the case. Thus it is not known where the measles virus resided during that period. It may be that, after a primary infection, resting lymphocytes or cells from the central nervous system harbor the virus. Indeed, it has been suggested that measles virus remains in the body of everyone because levels of antibodies against measles virus remain high during life, even in the absence of repeated antigenic stimulation from the environment (2). Measles virus has been isolated from human lymph nodes after recovery from the acute infection (3) and from lymph nodes of SSPE patients (4). Previously, we have reported that measles virus inhibits the stimulation of lymphocytes in vitro (5). This effect is caused by the infection of lymphocytes rather than of monocytes (6). Measles virus is known to be capable of infecting lymphocytes (7). However, it has been described that, for a productive infection, the lymphocytes must be stimulated (8, 9). In the present study the relation between lymphocyte stimulation and measles virus replication is further analyzed. As an explanation for the difference in susceptibility to virus infection between stimulated and nonstimulated lymphocytes, it has been suggested that measles virus is unable to enter nonstimulated cells, as though a necessary receptor is absent or masked (10). The present study shows that this is not so, but that the majority of nonstimulated lymphocytes do become infected. In the resulting infection, however, few of the apparently infected cells express demonstrable amounts of measles virus antigens. When the infected lymphocytes are subsequently stimulated with a mitogen, the measles virus is activated, resulting in the expression of measles virus antigen on most of the cells in the culture. With some exceptions virus replication is not sustained in nonstimulated lymphocytes. Stimulation by mitogens makes lymphocytes susceptible to virus replication. Studies with persistently infected mice show that stimulation of lymphocytes with mitogens leads to activation of latent C-type viruses (11). Osunkoya et al. have described the activation of measles virus in lymphocytes obtained from children who