FIG 1.

(A) Rationale for focusing on FeLV-B as the most likely zoonotic agent. The percentage of natural isolates containing each FeLV subgroup is given in parentheses. (B) Outline of the molecular structure of FeLV-B proviruses that are derived by recombination between FeLV-A and endogenous (en) FeLV-related proviruses in the feline genome. The hybridization probes (ExU3, EnvB) used to detect FeLV sequences in human cells are indicated. (C) The three basic patterns of susceptibility of cultured human cells to infection with FeLV-B. Cells were exposed to FeLV-B at various multiplicities of infection and were passaged for 14 days before the extraction of high-molecular-weight DNA and analysis of FeLV proviral DNA content by Southern blot hybridization of KpnI-digested DNA with the ExU3 probe. -ve, uninfected control. (D) Relative expression of mRNA encoding the FeLV-B receptor PIT-1 in a panel of cell lines sorted by the pattern of FeLV-B spread (phenotype 1, 2, or 3, as exemplified in panel C).