Feline Leukemia Virus Variants in Experimentally Induced Thymic Lymphosarcomas

Abstract

This study was initiated to evaluate the in vivo infectivity and pathogenicity of a group of recombinant feline leukemia viruses (rFeLVs) previously generated by in vitro forced recombination between a FeLV subgroup A virus (FeLV-A) and an endogenous FeLV (enFeLV) envelope (env) element (Sheets et al., 1992, Virology 190, 849-855). To determine infectivity of rFeLVs, neonatal cats were inoculated with rFeLVs alone or in combination with FeLV-A. The recombinant viruses were able to replicate efficiently in vivo only when administered along with FeLV-A. Of six co-infected cats, three developed thymic lymphosarcomas, one severe aplastic anemia, and two cachexia and depression; all were viremic and seroconverted shortly after inoculation. While both virus types were detected in virtually all tissues examined from these tumor-bearing cats, there was a particularly noteworthy sequence reversion in the rFeLVs. It is known that exogenous FeLV isolates carry a conserved neutralizing MGPNL epitope in the middle of the surface glycoprotein domain of the env gene. In contrast, the parental recombinant viruses used to inoculate these cats harbored the enFeLV-derived MGPNP sequence at this position. However, all in vivo-propagated recombinants displayed the MGPNL sequence, while the env-encoded backbone flanking the MGPNL sequence was that of the parental recombinant virus. These results suggest that viruses with the MGPNL epitope have an in vivo proliferative advantage. The data also provide an explanation for the conservation of this epitope in exogenous FeLVs despite the existence of variant forms in enFeLV proviral elements with which they can recombine.

A Novel Truncated env Gene Isolated from a Feline Leukemia Virus-Induced Thymic Lymphosarcoma

“…The 5Ј primer was made to the sequences conserved between FeLV-A and endogenous FeLV pol region (H18) (3), and the 3Ј primer was complementary to the exogenous 3Ј long terminal repeat (LTR) sequence of molecular clone FeLV-A, Rickard strain (FRA), or F6A (H20) (3). The env sequence was also amplified by using the same strategy from the tumor of cat 4746-5, which was challenged with an FeLV-A Rickard plasma preparation and a mixture of in vitro-generated recombinant FeLVs (14,24).…”

“…PCR products of full-length env genes from tumors 4746-5, 5022, 5023, 5024, and 5025 and the 700-bp deleted env gene from tumor 5023 were cloned into the pCR2.1 vector (Invitrogen). To study recombinant env species harbored by these exper-imental tumors, the complete env clones were then screened with the 5Ј primer RB53 (14) corresponding to the endogenous-related FeLV env sequence and the 3Ј primer RB17 (14) complementary to the FeLV-A env sequence.…”

“…Each sample was tested in triplicate. Filtered conditioned medium (25 l) was mixed with 25 l of a cocktail containing 50 nM Tris (pH, 8.3), 10 mM dithiothreitol, 10 mM MgCl 2 , 60 mM NaCl, 0.05% NP-40, 2 g of poly(rA-dT) [12][13][14][15][16][17][18] , and 3 Ci/ml ␣ 32 P-dTTP. After 2 h at 37°C, 5 l of the reaction mixture was transferred to 2-by-2-cm squares of DE81 chromatographic paper (Whatman International, Ltd.) and allowed to dry.…”

“…The viral species with specific adaptive amino acid mutations and with certain sites of recombination are rapidly selected for replication efficiency and are overrepresented at later time points after infection (2,3,14). FeLVs with recombinant env genes are detected with high frequency in naturally as well as experimentally induced feline lymphosarcomas (2,3,10,14,23,25). Evidence also exists to suggest that some defective env genes detected in FeLV-induced lymphosarcomas may be additional factors in the disease process (16,23).…”

“…There is solid evidence to indicate that interactions between infectious FeLV and non-infectious inherited endogenous FeLV elements generate recombinant viral quasispecies which represent a variety of chimeric envelope glycoproteins depending on the extent of amino terminal portion replaced by the endogenous env sequences (9,13,24). The viral species with specific adaptive amino acid mutations and with certain sites of recombination are rapidly selected for replication efficiency and are overrepresented at later time points after infection (2,3,14). FeLVs with recombinant env genes are detected with high frequency in naturally as well as experimentally induced feline lymphosarcomas (2,3,10,14,23,25).…”