Detection of antigenic heterogeneity in feline coronavirus nucleocapsid in feline pyogranulomatous meningoencephalitis.

A new monoclonal antibody (mAb), CCV2-2, was compared with the widely used FIPV3-70 mAb, both directed against canine coronavirus (CCoV), as a diagnostic and research tool. Western blot showed that both anti-CCoV mAbs only reacted with a protein of 50 kD, a weight consistent with the feline coronavirus (FCoV) viral nucleocapsid. A competitive inhibition enzyme-linked immunosorbent assay showed that the 2 recognized epitopes are distinct. Preincubation of CCV2-2 mAb with FCoV antigen suppressed the immunostaining. Formalin-fixed, paraffin-embedded sections from brains of 15 cats with the dry form of feline infectious peritonitis (FIP) were examined by immunohistochemistry. Immunohistochemistry was performed with both anti-CCoV mAbs, either on consecutive or on the same sections. A myeloid-histiocytic marker, MAC 387, was also used to identify FIP virus-infected cells. In all regions where MAC 387-positive cells were present, positive staining with the CCV2-2 mAb was systematically detected, except at some levels in 1 cat. In contrast, none or only a few cells were positive for the FIPV3-70 mAb. Double immunostaining showed macrophages that were immunopositive for either CCV2-2 alone or alternatively for CCV2-2 and FIPV3-70 mAbs. This reveals the coexistence of 2 cohorts of phagocytes whose FIP viral contents differed by the presence or absence of the FIPV3-70-recognized epitope. These findings provide evidence for antigenic heterogeneity in coronavirus nucleocapsid protein in FIP lesions, a result that is in line with molecular observations. In addition, we provide for the first time morphologic depiction of viral variants distribution in these lesions.

Pathologic, immunohistochemical, and electron microscopic findings in naturally occurring virulent systemic feline calicivirus infection in cats.

Infection with feline calicivirus (FCV) is a common cause of upper respiratory and oral disease in cats. FCV infection is rarely fatal, however, virulent, systemic strains of FCV (VS-FCV) that cause alopecia, cutaneous ulcers, subcutaneous edema, and high mortality in affected cats have recently been described. Seven cats with natural VS-FCV infection all had subcutaneous edema and ulceration of the oral cavity, with variable ulceration of the pinnae, pawpads, nares, and skin. Other lesions that were present in some affected cats included bronchointerstitial pneumonia, and pancreatic, hepatic, and splenic necrosis. Viral antigen was present within endothelial and epithelial cells in affected tissues as determined by immunohistochemical staining with a monoclonal antibody to FCV. Mature intranuclear and intracytoplasmic virions in necrotic epithelial cells were identified by transmission electron microscopy. VS-FCV infection causes epithelial cell cytolysis and systemic vascular compromise in susceptible cats, leading to cutaneous ulceration, severe edema, and high mortality.

Coronavirus-associated epizootic catarrhal enteritis in ferrets.

OBJECTIVE: To characterize clinical signs and lesions and identify the etiologic agent associated with epizootic catarrhal enteritis in domestic ferrets. DESIGN: Cross-sectional study. ANIMALS: 119 ferrets with epizootic diarrhea of presumed viral cause and 5 control ferrets. PROCEDURE: Clinical records and biopsy or necropsy specimens of ferrets with presumed epizootic catarrhal enteritis were reviewed. Immunohistochemical staining for coronavirus antigen was performed on paraffin-embedded tissues from approximately 10% of affected ferrets to identify viral antigen and determine its distribution. Transmission electron microscopy was performed on fecal samples and sections of jejunum. Virus isolation studies as well as immunofluorescent tests for other similar viruses were performed. RESULTS: Characteristic microscopic lesions consistent with intestinal coronavirus infection (vacuolar degeneration and necrosis of villus enterocytes; villus atrophy, fusion, and blunting; and lymphocytic enteritis) were consistently detected in affected ferrets. Coronavirus particles were identified in feces and jejunal enterocytes by use of transmission electron microscopy. Immunohistochemical staining of jejunal sections revealed coronavirus antigens. Antigen staining was not detected in healthy ferrets or ferrets with other gastrointestinal tract diseases. Virus isolation was unsuccessful, and other similar viruses were not detected. CONCLUSIONS AND CLINICAL RELEVANCE: Results strongly implicate a coronavirus as the causative agent of epizootic catarrhal enteritis in ferrets. Diagnosis may be made on the basis of a combination of historical, clinical, and microscopic findings.

Expression of viral proteins in feline leukemia virus-associated enteritis.

Fourteen cases of feline leukemia virus (FeLV)-associated enteritis were immunohistologically examined for the expression of FeLV proteins gp70, p27, and p15E in the jejunum, mesenteric lymph nodes, spleen, and bone marrow. Results were compared with those of FeLV-infected cats without intestinal alterations. Other viral infections and specific bacterial, fungal, and parasitic infections were excluded by standard microbiologic methods, histopathology, immunohistology, and in situ hybridization. In FeLV-associated enteritis, FeLV gp70 and p15E were strongly expressed in intestinal crypt epithelial cells. In contrast, FeLV-positive cats without intestinal alterations showed only faint staining for gp70 and p15E and comparatively strong p27 expression in these cells. Findings suggest a direct relation between FeLV infection and alterations in intestinal crypt epithelial cells that may be attributed to the envelope proteins gp70 and p15E and/or their precursor protein. Distinct similarities to the intestinal changes in the experimentally induced FeLV-feline AIDS syndrome are obvious, suggesting that naturally occurring feline AIDS variants may be responsible for FeLV-associated enteritis.

Feline immunodeficiency virus Vif localizes to the nucleus.

Monoclonal antibodies prepared against recombinant Vif derived from the 34TF10 strain of feline immunodeficiency virus (FIV) were used to assess the expression and localization of Vif in virus-infected cells. Analyses by Western blotting and by immunoprecipitation from cells infected with FIV-34TF10 revealed the presence of a single 29-kDa species specific for virus-infected cells. Confirmation of antibody specificity was also performed by specific immunoprecipitation of in vitro-transcribed and -translated recombinant Vif. Localization experiments were also performed on virus-infected cells, using different fixation procedures. Results for methanol fixation protocols similar to those reported for localization of human immunodeficiency virus (HIV) Vif showed a predominant cytoplasmic localization for FIV Vif, very similar to localization of HIV type 1 Vif and virtually identical to the localization observed for the Gag antigens of the virus. However, with milder fixation procedures that used 2% formaldehyde at 4 degrees C, FIV Vif was strongly evident in the nucleus. The localization was distinct from the nuclear localization noted with Rev and did not involve the nucleolus. Attempts to show colocalization or coprecipitation of Vif with Gag antigens were unsuccessful. In addition, Vif was not detected in purified FIV virions. The results are consistent with the notion that the primary role of Vif in virus infection initiates in the nucleus.

FIV infection of IL-2-dependent and -independent feline lymphocyte lines: host cells range distinctions and specific cytokine upregulation.

We have analyzed the ability of three molecular clones of feline immunodeficiency virus (FIV) and an ex vivo variant to infect nine distinct specific-pathogen-free feline cell lines in tissue culture. The purpose of these studies was to elucidate mechanisms by which host cells regulate the level of virus infection and expression and to assess host cell cytokine responses to virus infection. Cells used for the analyzes included four IL-2-dependent continuous T-cell lines (104-C1, 104-C7, MCH5-4 and DB FeTs) which arose from long-term passage, followed by limiting dilution cloning of peripheral blood mononuclear cells (PBMCs); two IL-2-independent T-cell lines (104-C1DL and MCH5-4DL) which originated from two of the IL-2-dependent lines, 104-C1 and MCH5-4; respectively; Crandell feline kidney cells (CrFK); G355-5 brain-derived glial cells; and the T-cell lymphoma line, 3201. Cells were infected with FIV-PPR, FIV-34TF10, FIV 34TF10orf2rep, and a variant arising from FIV-PPR during ex vivo passage on 104-C1DL cells, termed FIV-PPRglial. Infection of the IL-2-dependent T-cell line, 104-C1, by FIV-PPR resulted in the specific and distinct upregulation of cytokine expression. In particular, these cells doubled their expression of the pleiotropic cytokines, interleukin-4 and interleukin-12 after FIV infection. Interferon-gamma production also increased after infection with FIV whereas, TNFalpha expression remained constant. Also, a marked upregulation of MHC class II expression was noted post infection of MCH5-4 and 104-C1 cells with FIV-PPR. Similar results were obtained after infection with FIV-34TF10orf2rep, indicating that the upregulation of cytokine expression is not an isolate-specific phenomenon. Changes in cytokine and class II expression are similar to various reports for the in vivo cytokine alterations in FIV, SIV and HIV infections. The ex vivo infection of these cell lines offers amanipulable system to examine the mechanism(s) by which lentiviruses alter cytokine expression.

Feline immunodeficiency virus (FIV)-associated lymphoma: a potential role for immune dysfunction in tumourigenesis.

To determine the potential role of immune dysfunction in feline immunodeficiency virus (FIV)-associated lymphomagenesis, we present the results of immunological monitoring during the chronic phase of experimental FIV infection in two cats which subsequently developed lymphoma. In one cat, C1, cell-mediated immunity was depressed throughout the monitoring period but particularly from 125-200 weeks post-infection (pi), when this cat demonstrated profoundly impaired lymphocyte blastogenesis and markedly increased interleukin-1 (IL-1) production compared to age-matched, uninfected control cats. Lymphocyte function in the other cat, C2, was preserved to a greater degree. Alterations in the levels of immunoglobulin isotypes M, A and G in CD4+-, CD8+- and CD21+-lymphocyte sub-sets were demonstrated in both cats. Southern blot analysis revealed the presence of integrated FIV-provirus in tumour DNA from C2 but not C1 indicating a possible direct role for the virus in the former case only. In this study we have characterised, for the first time, the FIV-induced immune dysfunction in cats which developed lymphoma, demonstrating potential indirect mechanisms of tumourigenesis.

Fatal enteritis associated with coronavirus infection in cats.

This report describes five cases of naturally occurring feline coronavirus enteritis. The affected animals, aged 2 months to 7 years, had a clinical history of intestinal symptoms, including diarrhoea or vomiting, or both. They exhibited variable histological changes in the epithelium of the small intestine, ranging from degeneration of single cells and detachment of groups of cells from the villous tips to regenerative processes of the crypt epithelia. Post-mortem diagnosis was based on the immunohistochemical demonstration of coronavirus antigen within intestinal epithelial cells and on the electron microscopical demonstration of coronavirus particles in the faeces. In addition, one animal was immunohistochemically positive for antigens of feline leukaemia virus (FeLV) and exhibited intestinal changes consistent with FeLV-associated enteritis. Two cats were tested serologically for feline immunodeficiency antibodies, with negative results. The findings indicate that natural coronavirus infection is a potential cause of severe enteritis in juvenile and adult cats.

The Vif and Gag proteins of human immunodeficiency virus type 1 colocalize in infected human T cells.

The Vif protein of human immunodeficiency virus type 1 (HIV-1) and other lentiviruses is required for efficient replication in primary cells and certain immortalized cell lines in vitro and, in all likelihood, for the establishment of pathogenic infections in vivo. Current hypotheses concerning Vif's mechanism of action posit that it operates in virus-expressing cells during virion assembly, budding, or maturation such that released virions are modified in a manner that enables them to undergo productive infection in subsequent viral challenges. To gain further insight into the mechanism of action of lentivirus Vif proteins, we have performed a variety of in situ localization and biochemical fractionation studies using cells in which Vif is essential for efficient replication. Double-label immunofluorescence analyses of cells productively infected with HIV-1 or feline immunodeficiency virus revealed dramatic patterns of colocalization between Vif and the virally encoded Gag proteins. Subcellular fractionations of human T cells expressing HIV-1 Vif performed in the absence of any detergent demonstrated that greater than 90% of Vif is associated with cellular membranes. Additional purification using a continuous density gradient indicated that the majority of the membrane-bound Vif copurifies with the plasma membrane. Taken together, these observations suggest that lentivirus Vif and Gag proteins colocalize at the plasma membrane as virion assembly and budding take place. As a result, Vif is able to exert its modulatory effect(s) on these late steps of the virus life cycle.

Variable expression of major histocompatibility complex class II in the domestic cat.

This paper describes the characterisation of six independently produced monoclonal antibodies (mAbs) specific for non-polymorphic determinants of feline major histocompatibility complex (MHC) class II. One mAb is an anti-sheep class II which cross-reacts with the cat and five have been produced in response to immunisation with purified feline immunodeficiency virus. Despite their independent source all the mAbs have identical reactivities, immunoprecipitating two complex groups of polypeptides of M(r) 33 to 36.000 (MHC class II alpha chains) and M(r) 28 to 31,000 (MHC class II beta chains). Immunoblot analysis showed them to be beta chain-specific. One and two-dimensional electrophoresis revealed the complexity of feline class II mass and charge and implied the expression of multiple class II loci in the cat. Furthermore, it was demonstrated that distinct cell populations expressed a distinct range of class II variants. This suggesting either the differential expression or the distinct post-translational modification of lymphocytes from different sites. The mAbs have also been used for the detailed examination of the cellular distribution and tissue localisation of MHC class II in the cat.

Retroviral vector-transduced cells expressing the core polyprotein induce feline immunodeficiency virus-specific cytotoxic T-lymphocytes from infected cats.

The core polyprotein of feline immunodeficiency virus (FIV) was expressed in primary feline T-lymphocytes using a retroviral vector. These cells were used as antigen-presenting stimulator cells (APSC) for the in vitro induction of cytotoxic T-lymphocytes (CTL) from feline peripheral blood mononuclear cells (PBMC). CTL from 4 cats chronically infected with the Petaluma strain of FIV specifically lysed autologous FIV-infected targets in an MHC-restricted manner. The CD8 phenotype of more than 70% of the induced effector cells (97% for cells from one cat) was consistent with MHC class I-restricted cytotoxicity. In addition, it was possible to detect low levels of core polyprotein-specific lysis from effector cells of two of the FIV-infected cats. When observed, the level of lysis, measured as a percentage of specific 111In release, was lower for the transgenic gag-expressing targets than for FIV-infected targets. The difference in killing may reflect the low level of core CTL were not detected in either PBMC stimulated with cells transduced by a retroviral vector without the FIV gag sequence or PBMC from an uninfected cat stimulated with autologous transgenic APSC. The detection of FIV-specific CTL from infected cats following stimulation with transgenic APSC suggests a role for retroviral vectors in determining CTL specific for individual lentiviral proteins in protective immunity.

Feline immunodeficiency virus (FIV)-specific cytotoxic T lymphocytes from chronically infected cats are induced in vitro by retroviral vector-transduced feline T cells expressing the FIV capsid protein.

We have previously reported the presence of feline immunodeficiency virus (FIV)-specific, major histocompatibility complex (MHC)-restricted cytolytic T lymphocytes (CTL) in experimentally FIV-infected cats. However, the fine specificity of the CTL and the role of individual FIV proteins in inducing FIV-specific CTL responses remain unknown. In this study, we examined the in vitro induction and activity of FIV p24 capsid-specific CTL obtained from cats that had been experimentally infected with FIV Petaluma for 30 to 56 months. An amphotropic murine retroviral vector was used to generate transgenic primary feline T lymphoblasts that expressed the FIV capsid protein. When the autologous capsid-transduced T cells were used in vitro to stimulate CTL responses from peripheral blood mononuclear cells of chronically infected cats, MHC-restricted lysis of virus-infected target cells was observed. The majority of the CTL expressed CD8, and depletion of this population, but not CD4+ cells, effectively diminished the CTL activity. When the autologous capsid-transduced T cells were used as target cells, lysis by capsid-induced effectors was not observed. Analysis of capsid-transduced T cell clones revealed a variable and low level of capsid expression among the clones. This study demonstrates the potential for using retroviral vectors as a means of inducing CTL effector cells that will specifically kill lentivirus-infected cells during lentiviral infection.

Interaction of acute feline herpesvirus-1 and chronic feline immunodeficiency virus infections in experimentally infected specific pathogen free cats.

Cats with or without chronic feline immunodeficiency virus (FIV) infection were exposed to feline herpesvirus, type 1 (FHV-1). FIV infected cats became sicker than non-FIV infected cats and required more supportive treatment. However, there were no differences in the length of their illness or in the levels and duration of FHV-1 shedding. FHV-1 infection caused a transient neutrophilia at Day 7 with a rapid return to preinfection levels. The neutrophilia coincided with a transient lymphopenia that was accompanied by a decline in both CD4+ and CD8+ T-lymphocytes. A brief decrease in the CD4+/CD8+ T-lymphocyte ratio occurred at Day 14 in both FIV infected and non-infected cats. This decrease was mainly the result of an absolute and transient increase in CD8+ T-lymphocytes. CD4+ and CD8+ T-lymphocyte numbers and CD4+/CD8+ T-lymphocyte ratios returned to baseline within 4-8 weeks in both FIV infected and non-infected cats. FIV infected cats produced less FHV-1 neutralizing antibodies during the first 3 weeks of infection than non-FIV infected animals. The IgM FHV-1 antibody response was depressed in FIV infected cats whereas the IgG antibody response was unaffected. FHV-1 infection evoked a comparable transient loss of lymphocyte blastogenic responses to concanavalin A and pokeweed mitogen in both FIV infected and non-infected cats. However, response to pokeweed mitogen took longer to return to normal in FIV infected animals. Lymphocytes from FIV infected cats had a greater and more sustained proliferative response to FHV-1 antigen than non-FIV infected cats. The ongoing IgG antibody response to FIV was not affected by FHV-1 infection.

Biologically selected recombinants between feline leukemia virus (FeLV) subgroup A and an endogenous FeLV element.

In efforts to elucidate the proximal leukemogens that might be produced during a feline leukemia virus (FeLV) infection of cats, homologous recombinations between molecularly cloned exogenous and endogenous FeLV proviruses of known sequences were examined in cell cultures in vitro. A plasmid containing an infectious member of the most commonly occurring FeLV subgroup (FeLV subgroup A or FeLV-A) was coexpressed with noninfectious constructs containing the envelope (env) gene of an endogenously inherited FeLV-like feline genomic element in transfected feline fibroblasts. The viruses generated were selected for their ability to propagate in human cells which are resistant to infection by the parental ecotropic FeLV-A or the noninfectious endogenous constructs. An analysis of the recombinants thus derived identified a limited number of sites in the env gene which were preferentially utilized in the generation of recombinant FeLVs under the selection conditions used. These sites were clustered in the surface glycoprotein (SU) moiety of the env gene, and it appeared that most, but not all, of the SU gene product of FeLV-A, beginning from the N-terminus, can be replaced by sequences from an endogenous element, still allowing the virus to be biologically viable. In fact, these substitutions in the env gene expanded infectivity of the parental FeLV-A from ecotropic to polytropic cell tropism. Additionally, substitutions in the SU region yielded many recombinants in which a primary neutralizing pentapeptide epitope of FeLV-A was altered because of its variance in the endogenous element. In several of the recombinants, this sequence was also found to be frequently mutated. Consistent with the changes identified in this antibody-binding domain, the recombinant viruses were only weakly inhibited by a monoclonal antibody directed against this epitope, while FeLV-A was highly sensitive to neutralization.

Severe neutropenia associated with griseofulvin therapy in cats with feline immunodeficiency virus infection.

Griseofulvin administration was associated with the development of absolute neutropenia in six of seven (86%) cats with feline immunodeficiency virus (FIV) infection. The neutropenia was severe (less than 400 neutrophils/microliter) in four of the six affected cats, and one cat died from sepsis. Neutrophil counts returned to baseline values within 15 days after drug withdrawal in all surviving cats. No symptoms or hematologic abnormalities were observed in four normal (FIV-seronegative) cats treated with the same lot of griseofulvin at equivalent doses. Neutropenia recurred in two of two FIV-seropositive cats upon griseofulvin rechallenge. Cats with FIV infections appear to be at increased risk for griseofulvin-associated neutropenia. This phenomenon may be analogous to the increased frequency of antibiotic-induced neutropenias observed in humans infected with the human immunodeficiency virus.

Hematologic manifestations of feline immunodeficiency virus infection.

Studies were done on 53 cats with community-acquired infection with the feline immunodeficiency virus (FIV) to determine if hematologic abnormalities were comparable with those observed in patients seropositive for the human immunodeficiency virus (HIV). Nine cats were asymptomatic, 24 had clinical symptoms equivalent to AIDS-related complex (ARC), and 20 had AIDS-like disease. Hematologic abnormalities were detected in 75% (40 of 53) of FIV-seropositive cats, and multiple concurrent cytopenias were common. Anemia, lymphopenia, neutropenia, and thrombocytopenia occurred in 36%, 53%, 34%, and 8% of FIV-seropositive cats, respectively. Cytopenias were seen only in symptomatic (ARC or AIDS) cats. The occurrence of cytopenias and the distribution of clinical stages were similar in cats with concurrent feline leukemia virus (FeLV) infection and those with FIV alone, suggesting that these abnormalities were a direct consequence of FIV infection. In addition, abnormalities were noted in 72% of marrows from symptomatic cats and included hyperplasia of individual cell lineages and dysmorphic features. Our results demonstrate that the hematologic manifestations of FIV infection are strikingly similar to those reported in HIV-seropositive patients. Thus, FIV infection in cats is an excellent animal model to study the pathogenesis of blood and marrow abnormalities in AIDS, as well as to evaluate the hematologic toxicities of drug therapies.

Feline immunodeficiency virus and feline leukemia virus infections and their relationships to lymphoid malignancies in cats: a retrospective study (1968-1988).

Sera from 353 cats with naturally occurring feline leukemia virus (FeLV) infection were collected in Boston, Los Angeles, New York City, and Seattle between 1968 and 1988. These sera were retrospectively assayed by enzyme-linked immunosorbent assay for antibodies to feline immunodeficiency virus (FIV). Fifty-one (14.4%) of the FeLV-positive sera had antibodies to FIV, indicating dual oncovirus and lentivirus infections. FIV infections were confirmed by Western blot analysis, antibodies against the 15 and 27 kDa proteins being used as definitive markers. FIV infection was diagnosed in one cat sampled in 1968 and in eight other cats sampled before 1975 in New York City. Illnesses exhibited by coinfected cats were similar to those of cats infected with FeLV only. Two unrelated cats with multicentric fibrosarcomas were found to be simultaneously infected with FIV, FeLV, and feline sarcoma virus. FIV was less contagious than FeLV in 73 cats residing in an exposure household between 1977 and 1980 as determined by evaluation of sera collected sequentially. In this household, 15 resident cats became FeLV infected whereas no cats contracted FIV infection. Comparison of serologic results from 53 cats with leukemia/lymphoma and matched controls confirmed a strong correlation between FeLV viremia and leukemia/lymphoma. A significant correlation between FIV infection and lymphoproliferative malignancies was also found independent of FeLV infection.

Feline leukemia virus and feline immunodeficiency virus infections in a cat with lymphoma.

Lymphoma was diagnosed in a 7-year-old domestic cat found to be infected with FeLV and feline immunodeficiency virus (FIV). The cat was affected by chronic disorders suggestive of immunosuppression, including gingivitis, periodontitis, keratitis, and abscesses. Despite treatment, peripheral keratitis of the left eye progressed, resulting in uveitis, chronic glaucoma, and eventual corneal rupture. Microscopic retinal and optic disk pathologic processes also were suspected. Abnormal jaw movements that were believed to be indicative of neurologic disease were observed. Approximately 17 months later, the cat developed generalized lymphadenopathy, hepatosplenomegaly, and bilateral renomegaly. Lymphoblastic lymphoma and glomerulonephritis were diagnosed histologically. Manganese- and magnesium-dependent reverse transcriptase activity were detected in supernatants from lymph node and spleen mononuclear cell cultures, suggesting T-lymphocyte infection with FeLV and FIV.

Chronic leukopenia associated with feline immunodeficiency virus infection in a cat.

Leukopenia attributable to lymphopenia and neutropenia was detected over a 28-week period in a 12-year-old domestic cat infected with feline immunodeficiency virus (FIV). Mild normocytic, normochronic anemia also was evident. Platelet counts were normal, and serum biochemical values were unremarkable. Antibodies to FIV were detected in serum by use of immunofluorescence and immunoblot electrophoresis assays. Cytologic evaluation of bone marrow aspirates revealed normal cellular morphologic features, maturation, and myeloid-to-erythroid ratio. Normal marrow cellularity was determined histologically. There was, however, a significant (P less than 0.01) inhibition of colony-forming unit granulocyte/macrophage-derived progenitors when marrow cells were cultured in the presence of autologous serum, compared with that when marrow cells were cultured in the presence of serum obtained from clinically normal cats, thus suggesting the presence of a humoral inhibitory substance directed specifically at the granulocyte/macrophage lineage. These cell culture results were consistent with those reported for human beings with acquired immunodeficiency syndrome and neutropenia. Thus, FIV infection may be an excellent animal model in which to study human immunodeficiency virus and should be considered in the differential diagnosis of cats with chronic leukopenia.

Natural feline leukemia virus variant escapes neutralization by a monoclonal antibody via an amino acid change outside the antibody-binding epitope.

We have molecularly cloned a natural variant of feline leukemia virus subtype B. This isolate is unique in that it is not neutralized by a monoclonal antibody which neutralized all other feline leukemia virus isolates tested, including members of the A, B, and C subtypes. Western immunoblotting indicated that the monoclonal antibody was less able to bind to the gp70 of the resistant isolate (designated lambda B1) than to the gp70s of susceptible viruses. Nucleotide sequence analysis of the envelope gene of lambda B1 revealed a high degree of homology with the susceptible Snyder-Theilen, Gardner-Arnstein, and Rickard subtype B isolates, including the presence of a 5-amino-acid minimal binding epitope required for binding by the neutralizing monoclonal antibody. The only change within the vicinity of this epitope was in a single nucleotide, and this difference changed a proline residue to leucine three amino acids from the N terminus of the binding epitope. Competitive binding studies with synthetic peptides indicated that substitution of leucine for proline resulted in a 10-fold decrease in the ability of the peptide to compete for antibody binding to native antigen. The results are consistent with the interpretation that this amino acid change lowers the affinity of antibody binding, resulting in failure of the antibody to neutralize the variant virus.