Cold-adapted recombinants of influenza A virus in MDCK cells. I. Development and characterization of A/Ann Arbor/6/60 X A/Alaska/6/77 recombinant viruses.

Recombinant influenza viruses made at 25 and 33” in Madin-Darby canine kidney (MDCK) cells using the cold-adapted A/Ann Arbor/6/60 virus and the wild-type A/ Alaska/6/‘77 virus were biologically and genetically analyzed. Eight recombinants were separated into two phenotypic groups based on cold-adapted (ca) and temperature-sensitive (ts) markers: ca and ts, ca and non-m. The ca recombinants showed different degrees of cold adaptibility (DOCA) and different patterns of virus growth at 25’. All recombinants contained at most three genes from the cold variant A/Ann Arbor/6160 virus (triple-gene recombinant) and most contained two or one gene from the cold variant parent (double-gene and single-gene recombinants, respectively). Further, the same three genes, RNA2, RNA3, and RNA5 (NA) were the only ca A/Ann Arbor/6/60 genes found in the various recombinants. Two clones contained all three A/Ann Arbor/6/60 genes and were both cold-adapted (ca) and temperature-sensitive (ts). All other recombinant clones were ca and non-m, and contained RNA2 and/or RNA5 (NA). Each set of singlegene ca recombinants correlated with a different, but specific cold-adapted characteristic exhibited by their growth curves at 25”. Single-gene recombinants containing only the RNA2 of A/Ann Arbor/6/60 virus showed rapid growth early in infection and intermediate final virus yield (between the titer of virus yield for the ca A/Ann Arbor/6/60 virus and the wild-type A/Alaska/6/7’7 virus; while the single-gene recombinant containing only the RNA5 (NA) of A/Ann Arbor/6/60 virus showed slow growth early in infection, but a high final virus yield (equivalent to that of the ca A/Ann Arbor/6/60 parent). The double-gene recombinant containing both these genes showed both rapid growth early in infection and a high final virus yield. Thus, cold adaptation can be transferred to recombinant viruses by at least two independent genes each of which can confer the cold-adaptive property by its own pathway. The genetic basis for temperature sensitivity involves both RNA2 and RNA3, but remains ambiguous in the absence of a single-gene recombinant containing only RNA3 of the cold variant.