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Six macaques, apparently uninfected, following low-dose exposure to the pathogenic SIVmac251and SIVSME660by the mucosal route, were used in a pilot study to investigate whether infectability of ex vivolymph nodes could predict resistance and/or susceptibility to SIV infection in vivo.Of six macaques exposed to the less-pathogenic virus SIVMNE, four resisted viral infection. Analysis of the susceptibility of the PBMC of these four animals before SIVMNEchallenge indicated that all of them were resistant to infection by the SIVBK28isolate and, in three of them, this resistance was dependent on CD8+ T cells. Blocks of lymph nodes of these four macaques were resistant to SIVMNEinfection ex vivofollowing SIVMNEviral challenge exposure. However, the same blocks from the same animals were permissive to the more virulent SIV251(32H). Accordingly, three of these macaques were readily infected following challenge exposure with SIV251(32H). Lymphoproliferative responses in blood or lymph nodes, local C-C chemokine production in the lymph-node explants, and cytotoxic T-cell activity measured throughout the study did not correlate with ex vivoresistance or susceptibility to in vivoinfection. In conclusion, PBMC and lymph-node resistance or susceptibility to infection ex vivoappeared to correlate with in vivoinfectivity and, thus, these approaches should be further tested for their predictive value for in vivoinfection.

Feline immunodeficiency virus (FIV) is a T-lymphotropic retrovirus associated with immunodeficiency and opportunistic infections in cats. The discovery of FIV provides an opportunity for the development of a small animal model for AIDS. To initiate the molecular and biological characterization of FIV, cDNA clones were synthesized and used to isolate a proviral clone of FIV. Molecular cross-hybridization analysis of FIV with five lentiviruses revealed that nucleotide-sequence similarities exist between FIV and these lentiviruses in the gag-pol genes. However, nucleotide-sequence similarities were not seen upon comparison of the FIV long terminal repeat sequence with known viral sequences. Common antigenic determinants appeared to be shared by FIV, caprine arthritis encephalitis virus, and visna virus as shown by serological cross-reactivity of rabbit antibodies to caprine arthritis encephalitis virus and visna virus with the putative FIV core protein p28. These studies demonstrated that FIV is a member of the lentivirus subfamily and is distantly related to the AIDS lentiviruses of primates. Importantly, progeny virions of our molecular clone were infectious for experimentally inoculated cats. The availability of an infectious molecular clone will make possible a detailed dissection of the molecular pathogenesis of FIV, which may facilitate the development of vaccine and therapeutic strategies for AIDS.

Simian immunodeficiency virus (SIV) and human immunodeficiency virus share the property of tropism for CD4-bearing cells. Infection is initiated by a high-affinity interaction between CD4 and conserved domains on the viral envelope glycoprotein. In this report, we demonstrated that SIV had a restricted host range among human CD4+ cells when compared with human immunodeficiency virus type 1 or type 2. This restricted tropism was associated with the inability of the SIV envelope glycoprotein to induce membrane fusion in cells not susceptible to productive exogenous infection by SIV. We conclude that the major route of SIV entry into CD4+ cells is by envelope-mediated direct fusion with the cell and that additional envelope-cell interactions after CD4 binding are required for productive infection.

A myc-containing recombinant feline leukemia provirus, designated FTT, was molecularly cloned from the cat T-cell lymphoma line F422. Its transforming activity, as well as the nucleotide sequence of the 3' 2.7 kilobases of FTT, including v-myc, was determined. The predicted v-myc protein differs from feline c-myc by three amino acid changes and is truncated by two amino acids at the carboxyl terminus. Comparison with feline leukemia virus (FeLV), feline c-myc, and other FeLV proviruses indicates that recombination junctions involved in the generation of FeLV-onc viruses occur at preferred locations within the virus. They usually follow or occur within the sequence ACCCC at 5' junctions and may result from homologous recombination between sequences of marked purine-pyrimidine strand bias, especially at 3' junctions. Some recombination sites also resemble recombinase recognition sequences utilized in immunoglobulin and T-cell receptor variable-region joining. Transfection of primary rat embryo fibroblasts and subsequent in vivo analysis revealed that morphologic and tumorigenic transformation require cotransfection of FTT with human EJ-ras DNA; neither gene alone is sufficient. FTT v-myc is expressed in these transformed rat cells as a 3.0-kilobase subgenomic RNA; however, in contrast to the depressed level of c-myc expression in v-myc-involved feline tumors, steady-state levels of rat c-myc RNA and protein are apparently unaltered.

We report the first complete nucleotide sequence (8,440 base pairs) of a biologically active feline leukemia virus (FeLV), designated FeLV-61E (or F6A), and the molecular cloning, biological activity, and env-long terminal repeat (LTR) sequence of another FeLV isolate, FeLV-3281 (or F3A). F6A corresponds to the non-disease-specific common-form component of the immunodeficiency disease-inducing strain of FeLV, FeLV-FAIDS, and was isolated from tissue DNA of a cat following experimental transmission of naturally occurring feline acquired immunodeficiency syndrome. F3A clones were derived from a subgroup-A-virus-producing feline tumor cell line. Both are unusual relative to other molecularly cloned FeLVs studied to date in their ability to induce viremia in weanling (8-week-old) cats and in their failure to induce acute disease. The F6A provirus is organized into 5'-LTR-gag-pol-env-LTR-3' regions; the gag and pol open reading frames are separated by an amber codon, and env is in a different reading frame. The deduced extracellular glycoproteins of F6A, F3A, and the Glasgow-1 subgroup A isolate of FeLV (M. Stewart, M. Warnock, A. Wheeler, N. Wilkie, J. Mullins, D. Onions, and J. Neil, J. Virol. 58:825-834, 1986) are 98% homologous, despite having been isolated from naturally infected cats 6 to 13 years apart and from widely different geographic locations. As a group, their envelope gene sequences differ markedly from those of the disease-associated subgroup B and acutely pathogenic subgroup C viruses. Thus, F6A and F3A correspond to members of a highly conserved family and represent prototypes of the horizontally transmitted, minimally pathogenic FeLV present in all naturally occurring infections.

We determined the complete nucleotide sequence of an infectious proviral molecular clone (FIV-14) of the feline immunodeficiency virus (FIV). FIV-14 has a genome organization similar in complexity to other lentiviruses. In addition to three large open reading frames representing the gag, pol, and env genes, at least four small open reading frames are present in the pol-env intergenic, env, and env-3' long terminal repeat regions. Nucleotide and deduced amino acid sequence alignments of the FIV coding sequences with analogous sequences of other lentiviruses revealed significant identities only in the gag and pol genes. Phylogenetic tree analyses of gag and pol gene-encoded protein sequences demonstrate that FIV is more closely related to the ungulate lentiviruses, equine infectious anemia virus and visna virus, than to the primate lentiviruses, human and simian immunodeficiency viruses.

Simian T-lymphotropic retroviruses with structural, antigenic, and cytopathic features similar to the etiologic agent of human acquired immunodeficiency syndrome, human T-lymphotropic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV), have been isolated from a variety of primate species including African green monkeys (STLV-IIIAGM). This report describes nucleic acid cross-reactivity between STLV-IIIAGM and HTLV-III/LAV, molecular cloning of the STLV-IIIAGM genome, and evaluation of its structure and genetic relationship to other retroviruses. Overlapping clones from a cell line infected with virus from a single animal were found to encompass the entire STLV-IIIAGM genome and exhibit a limited degree of restriction-site variability. Specific hybridizing fragments were detected in DNA from this and other STLV-IIIAGM-infected cell lines. A fraction of viral DNA present in at least two STLV-IIIAGM lines persists as unintegrated viral DNA, a characteristic of infection with cytopathic retroviruses. Strongest cross-reactivity was detected between HTLV-III/LAV pol- and gag- genes and STLV-IIIAGM, whereas no cross-reactivity was detected between STLV-IIIAGM and molecular clones of human T-lymphotropic virus types I and II (HTLV-I and -II), visna virus, bovine leukemia virus, or feline leukemia virus.