Nonstructural c protein is required for efficient measles virus replication in human peripheral blood cells

The P gene of measles virus (MV) encodes the phosphoprotein, a component of the virus ribonucleoprotein complex, and two nonstructural proteins, C and V, with unknown functions. Growth of recombinant MV, defective in C or V e xpression, was explored in human peripheral blood mononuclear cells (PBMC). The production of infectious recombinant MV V- was comparable to that of p arental MV tag in simian Vero fibroblasts and in PBMC. In contrast, MV C- p rogeny was strongly reduced in PBMC but not in Vero cells. Consistently, th e expression of both hemagglutinin and fusion proteins, as well as that of nucleoprotein mRNA, was lower in MV C--infected PBMC. Thus, efficient repli cation of MV in natural host cells requires the expression of the nonstruct ural C protein. The immunosuppression that accompanies MV infection is asso ciated with a decrease in the in vitro lymphoproliferative response to mito gens. MV C- was as potent as MV tag or MV V- in inhibiting the phytohemaggl utinin-induced proliferation of PBMC, indicating that neither the C protein nor the V protein is directly involved in this effect.