Murine gammaherpesvirus 68 encodes a functional regulator of complement activation

Sequence analysis of the murine gammaherpesvirus 68 (gamma HV68) genome rev ealed an open reading frame (gene 4) which is homologous to a family of pro teins known as the regulators of complement activation (RCA proteins) (H. W . Virgin, P. Latreille, P. Wamsley, K. Hallsworth, K. E. Weck, A. J. Dal Ca nto, and S. N. Speck, J. Virol, 71:5894-5904, 1997). The predicted gene 4 p roduct has homology to other virally encoded RCA homologs, as well as to th e complement-regulatory proteins decay-accelerating factor and membrane cof actor protein. Analyses by Northern blotting and rapid amplification of cDN A ends revealed that gene 4 is transcribed as a 5.2-kb bicistronic transcri pt of the late kinetic class. Three gamma HV68 RCA protein Isoforms (60 to 65 kDa, 50 to 55 kDa, and 40 to 45 kDa) were detected by Western blotting o f infected murine MN 3T12 fibroblast cells. A soluble 40- to 45-kDa isoform was detected in the supernatants of virally infected cells. Flow cytometri c analysis revealed that the gamma HV68 RCA protein was expressed on the su rfaces of infected cells. Supernatants from virally infected cells containe d an activity that inhibited murine complement activation as measured by in hibition of C3 deposition on activated zymosan particles. Recombinant gamma HV68 RCA protein, containing the four conserved short consensus repeats, i nhibited murine C3 deposition on zymosan via both classical and alternative pathways and inhibited deposition of human C3 on activated zymosan particl es. Expression of this inhibitor of complement activation, both at the cell surface and in the fluid phase, may be important for gamma HV68 pathogenes is via the inhibition of innate and adaptive immunity.