Isotypically restricted activation of B lymphocytes by lactic dehydrogenase virus

Lactic dehydrogenase elevating virus (LDV) was found to selectively stimulate IgG2a synthesis in infected mice. Within one week after infection, the production of IgG2a increased nearly 50-fold whereas that of IgM, IgA, IgG1 and IgG3 remained virtually unchanged. IgG2b synthesis was also enhanced but to a lesser extent. Several observations suggested that this stimulation of IgG2 production resulted from a polyclonal B cell activation: (a) the isoelectric focusing patterns of IgG2a before and after LDV infection were exactly the same, (b) the frequency of clones with anti-LDV activity in hybridoma collections derived from infected mice was extremely low (less than 4/1000) and (c) the proliferative response elicited by LDV in unsensitized animals was comparable with that induced by lipopolysaccharide. The effect of LDV on immunoglobulin synthesis was drastically reduced in nude mice but was not affected by the X-linked B lymphocyte defect of animals carrying the xid mutation.