Complementation of a herpes simplex virus type 1 Vmw110 deletion mutant by human cytomegalovirus

The herpes simplex virus type 1 (HSV-1) mutant dl1403 contains a 2 kb deletion within the sequences encoding the immediate early polypeptide Vmw110. Previous experiments showed that although dl1403 exhibits normal patterns of gene expression following infection at an m.o.i. of 5 p.f.u./cell its growth and plaquing efficiency are impaired in low multiplicity infections, particularly in human foetal lung (HFL) cells. We have now investigated the ability of two other human herpesviruses, varicella-zoster virus (VZV) and human cytomegalovirus (HCMV), to compensate for this defect at low m.o.i. in HFL cells. Co-infection with HCMV resulted in greatly increased plaque numbers and the apparent particle/p.f.u. ratios of dl1403 stocks were reduced to values similar to those exhibited by wild-type HSV-1 stocks. Complementation of dl1403 in low multiplicity infections by HCMV and VZV was also demonstrated by an increased yield of the mutant virus and an increase in synthesis of dl1403 DNA. Ultraviolet irradiation of HCMV abolished its ability to complement dl1403 and the presence of adenovirus 5 had no stimulatory effect on dl1403 DNA replication. When HFL monolayers were infected with dilutions of dl1403 stocks such that no plaques were produced, replication of the mutant virus could be induced by superinfection with HCMV 7 days after the initial infection. These results indicate that a non-lytic interaction between dl1403 and HFL cells is a more likely consequence of a low multiplicity infection than plaque formation.