Attenuated live viral strains have afforded significant protection against virus challenge in HIV vaccine models. Although both cellular and humoral immunity are assumed to be vital for protection, specific parameters consistently associated with control of infection have been elusive. Our previous studies have shown that lentiviruses from 2 nondomestic feline species--lion (Pathera leo) and puma (Felis concolor)--persistently but nonpathogenetically infect domestic cats (Felis domestica). Moreover, infection with either the puma lentivirus (PLV) or lion lentivirus (LLV) conferred partial protection against superinfection with virulent feline immunodeficiency virus (FIV), the feline equivalent of HIV. To determine whether domestic cats infected by the lentiviruses of pumas or lions generate cross-reactive immune responses, we infected groups of 5 domestic cats with PLV, LLV, or a sham control and then monitored virus load, hematologic parameters, antibody protection, proliferative responses, and the ability of blood mononuclear cells to inhibit LLV, PLV, and FIV replication in vitro. All cats inoculated with LLV or PLV developed persistent infection, and low-level cell-associated viremia has been previously described. Infected cats also generated robust antibody titers and lymphocytes that proliferated in response to viral antigens and downregulated PLV, LLV, and FIV replication in vitro. This latter activity was CD8 cell associated for PLV and LLV inhibition but not for FIV inhibition. Thus, cats infected with the phylogenetically more ancient and less pathogenic feline lentiviruses generated humoral and cell-mediated immune responses reactive against both the homologous viruses and the heterologous FIV of domestic cats, which correlated with decreased viral load. These results are analogous to protection studies with attenuated primate immunodeficiency viruses and provide a system by which to examine adaptation, interference, and cross protection among lentiviruses.