The immunobiology of the feline leukemia virus

Feline Leukemia Virus p27 Antigen Concentration and Proviral DNA Load Are Associated with Survival in Naturally Infected Cats

Longitudinal studies of cats naturally infected with feline leukemia virus (FeLV) are important for understanding disease outcomes. Levels of p27 antigen and copy numbers of proviral DNA have been associated with FeLV-infection courses. The purpose of this prospective study was to establish cutoff values for p27 antigen concentration and proviral DNA load that distinguished high positive from low positive groups of cats and to evaluate an association with survival. At enrollment, 254 cats were tested by point-of-care and microtiter plate enzyme-linked immunosorbent assays (ELISAs) for p27 antigen and real-time polymerase chain reaction (PCR) for proviral DNA. The 127 positive cats were retested monthly for six months and monitored for survival over the four-year study. A receiver operating characteristic-based analysis of samples with concordant or discordant qualitative results for p27 antigen and proviral DNA was used to establish cutoff values, and when applied to test results at enrollment for classifying cats as high positive or low positive, a significant difference in survival was observed. High positive cats had a median survival of 1.37 years (95% CI 0.83-2.02) from time of enrollment, while most low positive cats were still alive (93.1% survival). Quantitative results for p27 antigen concentration and proviral DNA load were highly correlated with survival times in FeLV-infected cats.

Evaluation of a quantitative enzyme-linked immunosorbent assay for feline leukemia virus p27 antigen and comparison to proviral DNA loads by real-time polymerase chain reaction

Feline leukemia virus (FeLV) is an oncogenic retrovirus of cats. While higher viral RNA and proviral DNA loads have been correlated with progressive infections and disease, a similar correlation has been suggested for p27 antigen concentrations. This analytical study compared the results of a quantitative ELISA for p27 antigen with quantitative real-time PCR results for FeLV proviral DNA in patient samples. A significant positive correlation between copies of proviral DNA and the concentration of p27 antigen was identified (r = 0.761, P < 0.0001). Samples with high proviral DNA loads, at least 1 × 10⁶ copies/mL of whole blood, typically had p27 antigen concentrations greater than 30 ng/mL in plasma. Samples with proviral DNA loads below this level all had concentrations of p27 antigen in plasma that were less than 10 ng/mL. Given this correlation, it is hypothesized that the concentration of p27 antigen at a given point in time may help to indicate the likelihood of a progressive or regressive infection similar to what has been demonstrated for proviral DNA loads.

Multiple Introductions of Domestic Cat Feline Leukemia Virus in Endangered Florida Panthers

The endangered Florida panther (Puma concolor coryi) had an outbreak of infection with feline leukemia virus (FeLV) in the early 2000s that resulted in the deaths of 3 animals. A vaccination campaign was instituted during 2003-2007 and no additional cases were recorded until 2010. During 2010-2016, six additional FeLV cases were documented. We characterized FeLV genomes isolated from Florida panthers from both outbreaks and compared them with full-length genomes of FeLVs isolated from contemporary Florida domestic cats. Phylogenetic analyses identified at least 2 circulating FeLV strains in panthers, which represent separate introductions from domestic cats. The original FeLV virus outbreak strain is either still circulating or another domestic cat transmission event has occurred with a closely related variant. We also report a case of a cross-species transmission event of an oncogenic FeLV recombinant (FeLV-B). Evidence of multiple FeLV strains and detection of FeLV-B indicate Florida panthers are at high risk for FeLV infection.

Disentangling the link between supplemental feeding, population density, and the prevalence of pathogens in urban stray cats

BACKGROUND

Supplemental feeding of free-roaming animals, including wildlife and feral or stray animals, is well known to have a substantial impact on various aspects of animal ecology including habitat use, activity patterns, and host-pathogen interactions. Among them, an increased population density (PD) of animals receiving supplemental food raises concerns regarding the transmission of pathogens in these host populations. The primary aim of this study was to investigate how supplemental feeding is associated with host PD and prevalence of pathogens with different transmission modes in urban stray cats. We hypothesized that supplemental feeding would be positively associated with host PD and the prevalence of pathogens with density-dependent transmission modes compared with pathogens with transmission modes that are considered relatively density-independent.

METHODS

This study was conducted in six districts in Seoul, Republic of Korea which were selected based on different degrees of supplemental feeding and cat caretaker activity (CCA). The PD of stray cats was estimated by mark-recapture surveys. Stray cat blood samples (N = 302) were collected from stray cats by local animal hospitals from each district performing the trap-neuter-release which tested for eight pathogens with different transmission modes (feline immunodeficiency virus, feline leukemia virus (FeLV), feline panleukopenia virus, feline calicivirus, feline herpesvirus-1, Bartonella henselae, hemoplasma, and Toxoplasma gondii) with molecular or serological assays. Associations between the prevalence of each pathogen and PD, CCA, and sex of cats were statistically analyzed.

RESULTS

In contrast to initial predictions, the cat PD was generally higher in low CCA districts. The prevalence of (FeLV), which is transmitted through direct contact, was significantly higher in areas with a high CCA, conforming to our hypothesis. On the other hand, the prevalence of feline parvovirus, which can be spread by environmental transmission, was higher in low CCA districts. The remaining six pathogens did not show any association with the CCA; however, they had a unique association with the PD or the sex of the stray cats.

DISCUSSION

Our findings suggest that in addition to influencing the PD, supplemental feeding may affect the prevalence of pathogens in urban animals by mechanisms such as increased aggregation and/or altered foraging strategies, with different consequences depending on the transmission mode of each pathogen.

Too much of a good thing: resource provisioning alters infectious disease dynamics in wildlife

Provisioning of abundant food resources in human-altered landscapes can have profound effects on wildlife ecology, with important implications for pathogen transmission. While empirical studies have quantified the effects of provisioning on host behaviour and immunology, the net interactive effect of these components on host-pathogen dynamics is unknown. We use simple compartmental models to investigate how provisioning-induced changes to host demography, contact behaviour and immune defence influence pathogen invasion and persistence. We show that pathogen invasion success and equilibrium prevalence depend critically on how provisioning affects host immune defence and that moderate levels of provisioning can lead to drastically different outcomes of pathogen extinction or maximizing prevalence. These results highlight the need for further empirical studies to fully understand how provisioning affects pathogen transmission in urbanized environments.

High prevalence of non-productive FeLV infection in necropsied cats and significant association with pathological findings

Applying a combination of semi-nested PCR and immunohistology (IHC), the presence of exogenous feline leukemia virus infection was studied in 302 necropsied cats with various disorders. 9% showed the classical outcome of persistent productive FeLV infection which was represented by FeLV antigen expression in different organs. 152 cats (50%) harboured exogenous FeLV-specific proviral sequences in the bone marrow but did not express viral antigen. These cats were considered as horizontally but non-productively infected.

Statistical evaluation showed a significant association of non-productive horizontal FeLV infection with a variety of parameters. Non-productively infected cats were statistically significantly older and more often originated from animal shelters than cats without exogenous FeLV infection. Furthermore, some pathological disorders like anemia, panleukopenia, and purulent inflammation showed significant association with non-productive FeLV infection. No significant association was found with lymphosarcoma, known for a long time to be induced by productive FeLV infection.

Evaluation of a novel nested PCR for the routine diagnosis of feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV)

Laboratory diagnosis of feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) usually involves both viruses, as the clinical signs are similar and coinfection may occur. Serological methods may not represent an accurate diagnosis: maternal antibodies or cross-reactions may give false positive results to FIV, and false negative results may occur in latent FeLV status, or in certain FIV infection stages. A nested polymerase chain reaction (PCR) technique was designed to detect FeLV, FIV and feline endogenous retrovirus simultaneously. The detection of endogenous sequences was considered indicative of successful DNA extraction. The technique was used to diagnose FIV and FeLV in the blood cells of 179 cats. The kappa value with the serological data was 0.69 for FeLV and 0.87 for FIV. The joint detection of FeLV and FIV by this novel nested PCR is sensitive, specific, fast and convenient, and its applicability for clinical diagnosis is promising, as the direct evidence of the presence of the virus is more realistic than the indirect data provided by the serological detection.

Feline vaccine-associated fibrosarcoma: an ultrastructural study of 20 tumors (1996-1999)

Twenty feline vaccine-associated sarcomas were examined by transmission electron microscopy. Tumors contained pleomorphic spindle cells, histiocytoid cells, and giant cells. Most tumors contained myofibroblasts, which had morphologic features similar to those of fibroblasts. These cells were further distinguished by subplasmalemmal dense plaques and thin cytoplasmic actin myofilaments organized as elongated bundles concentrated at irregular intervals forming characteristic dense bodies. Intracellular crystalline particulate material was found in 5 of the 20 tumors. Energy dispersive X-ray spectroscopy was used to identify the crystalline material within one tumor as aluminum-based. One tumor from a feline leukemia virus-infected cat contained budding and immature retroviral particles.

Feline leukaemia virus: a review of immunity and vaccination

The availability feline leukaemia virus (FeLV) vaccines has added a new and important dimension to the control of this infectious agent. FeLV vaccination is a controversial issue, however, partly because of differences in the formulation between the current products, partly because of conflicting claims by vaccine manufactures and partly because experimental trials have shown that none of the vaccines provides 100 per cent protection against infection. This paper reviews the role of the immune response in determining the outcome following exposure to FeLV and describes the importance of FeLV subgroups. The five commercial FeLV vaccines currently available in the USA and Europe are described and the published literature on efficacy studies is summarised. However, these efficacy studies are often difficult to interpret for various reasons, including the small numbers of animals used; differences in challenge methods, vaccine strains and vaccine dose employed; and differences in postchallenge monitoring protocols.

Immunohistochemical identification of B and T lymphocytes in formalin-fixed, paraffin-embedded feline lymphosarcomas: relation to feline leukemia virus status, tumor site, and patient age

The lymphocyte phenotype of 70 formalin-fixed, paraffin-embedded feline lymphosarcomas (LSAs) was determined immunohistochemically using a T cell polyclonal antibody, and a B cell monoclonal antibody. Forty-seven of 70 (67%) tumors were T cell, 19/70 (27%) were B cell, and 4/70 (6%) did not stain with either marker. Thirty-eight of 70 (54%) tumors were positive for feline leukemia virus (FeLV) antigen by immunohistochemistry (IHC), and 52/70 (74%) tumors were positive for FeLV DNA using the polymerase chain reaction (PCR). B cell tumors were as frequently FeLV-positive as T cell tumors using either IHC or PCR. Intestinal tumors were more likely to be B cell than T. The incidence of B and T cell tumors was not different among young (< or = 3 y), middle-aged (> 3 y to < or = 8 y), and old (> 8 y) cats. Both B and T cell tumors from old cats were FeLV-positive more often by PCR than by IHC. Feline leukemia virus DNA but not antigen, was detected in B cell tumors and intestinal tumors from cats > 8 y as often as it was detected in B cell tumors and intestinal tumors from cats < or = 8 y. Previously, most B cell and intestinal tumors from old cats were considered to be negative for FeLV. Here, the results suggest involvement of latent or replication-defective forms of the virus in such tumors from old cats. This study supports a role for FeLV in feline B cell as well as T cell tumorigenesis.

Feline leukemia virus detection by ELISA and PCR in peripheral blood from 68 cats with high, moderate, or low suspicion of having FeLV-related disease

Clinicopathologic criteria were used to group 68 cats according to high, moderate, or low suspicion of having feline leukemia virus (FeLV)-related disease. Peripheral blood samples were tested for FeLV antigen by enzyme-linked immunosorbent assay (ELISA) and for FeLV DNA by polymerase chain reaction (PCR). There was no significant difference between ELISA and PCR results in the 68 cats. In the high-suspicion group, 46%(11/24) of cytopenic cats were test positive (ELISA and PCR) and 87% (13/15) with hemopoietic neoplasms were test-positive. Also within the high suspicion group, test-positive cats were 2.5 times more likely to die within the 1 year follow-up period than were test-negative (ELISA and PCR) cats. Among cats in the moderate-suspicion group, 15% (2/13) were test-positive, and none (0/16) of the cats in the low suspicion group was test positive. The relative risk of a positive test (ELISA and PCR) in the high suspicion group was 3.7 times that for the moderate-suspicion group and 22.8 times that for the low suspicion group. There was no significant difference in the relative risk of a positive test result between the moderate and low suspicion groups. The results indicate that FeLV detection by PCR can be adapted for diagnostic purposes using peripheral blood samples, however, results do not differ significantly from FeLV ELISA results. Also, a proportion of cats with a high suspicion of having FeLV-related cytopenia and hemopoietic tumors are negative for both circulating FeLV antigen and DNA. These cats may not have FeLV-related disease, or FeLV may exist in a disease-producing but nonreplicating form ultimately detectable by PCR in tissues other than peripheral blood.

A reverse transcription-polymerase chain reaction technique to detect feline cytokine genes

The ability to detect feline cytokine expression would allow further characterization of the feline immune system. Bioassays are currently available for the measurement of feline IL2, IL6 and TNF alpha but not for other biologically important cytokines. To detect the expression of other cytokines, a reverse transcription-polymerase chain reaction (RT-PCR) procedure was developed. Since feline cytokine gene sequences other than TNF alpha were not available, mammalian DNA and mRNA sequences for IL2, IFN gamma, IL4, IL6, IL10, IL12 and beta-actin, obtained from the Genbank database were compared and oligonucleotide primers chosen from consensus sequences. To validate the cytokine and beta-actin primers, peripheral blood mononuclear cells from specific pathogen free (SPF) cats were cultured in the presence of Con A for various periods of time (0-72 h). RNA was collected, reverse transcribed into cDNA, and the cDNA was amplified by PCR with each set of cytokine primer pairs. RT-PCR products were hybridized with specific 32P end-labeled internal oligonucleotide probes and then analyzed with the AMBIS imaging system to determine the kinetics of cytokine mRNA production. The beta-actin signal was used to control for sample to sample variation in the quantity of mRNA and variation in the RT and PCR reactions. Peak mRNA expression for most cytokines was found to occur between 2 to 4 h of Con A stimulation. mRNA expression was correlated with cytokine bioactivity for IL2 and IL6. Peak IL2 bioactivity occurred after 8 h of Con A stimulation, 4 h after the mRNA expression had peaked. Although IL6 mRNA expression peaked between 2 and 4 h of stimulation, bioactivity was not detected until 8 h of stimulation and continued to increase over the next 24-48 h.

Elevation of feline interleukin 6-like activity in feline immunodeficiency virus infection

The involvement of feline interleukin 6 (IL-6)-like activity in polyclonal B-cell activation in feline immunodeficiency virus (FIV) infection was investigated by using the proliferative response of hybridoma cell clone B3B1. Cats spontaneously infected with FIV had hyperimmunoglobulinemia, as shown by increases in the percentage of gamma-globulin and the plasma IgG concentration and decrease in the albumin/globulin (A/G) ratio. Because IL-6 plays an essential role in the differentiation of activated B cells into Ig-secreting cells, we examined the effect of FIV infection on the plasma IL-6 level. Plasma IL-6-like activity was found to be significantly higher in FIV-infected cats than in healthy controls. Peripheral blood mononuclear cells (PBMC) isolated from FIV-infected cats and cultured without any exogenous activators of IL-6 production released more feline IL-6-like activity than cells from healthy controls. This phenomenon was mainly due to the increase in the production of IL-6 by adherent cells such as monocytes/macrophages, but also partly by nonadherent cells. These results indicate that elevation of feline IL-6-like activity is associated with FIV infection and that overproduction of IL-6 may contribute to the polyclonal B-cell activation seen in FIV infection.

Immune mechanisms in the pathogenesis of viral diseases: a review

Three immunopathological mechanisms may determine the pathogenesis of viral diseases in animals. (1) A variety of viruses causes transient or prolonged immunosuppression by infecting lymphoreticular tissues and interacting with components of the immune system. (2) In persistent viral infections effective immune responses may result in tissue damage. The mechanisms involved are T-cell-mediated destruction of infected cells and delayed-type hypersensitivity. (3) In a number of viral diseases pathogenic immune complexes are formed when antibodies are produced and react with viral antigen molecules persisting in the host. The selected examples of immune dysfunction are the focus of this review.

Selective impairment of humoral immunity in feline leukemia virus-induced immunodeficiency

We used a panel of in vitro assays to investigate the nature of immune dysfunction in cats infected with FeLV-FAIDS, a naturally occurring, molecularly cloned feline leukemia virus (FeLV) isolate which induces a fatal immunodeficiency syndrome in infected cats. During the asymptomatic period preceding immunodeficiency disease, we were unable to detect any deficits in concanavalin A-induced blastogenesis, xenogeneic mixed-lymphocyte reaction assays, stimulation of lymphocytes by soluble protein antigen, and cytotoxic T lymphocyte assays. However, during this period humoral immune responses in the FeLV-FAIDS-infected cats were dramatically impaired. As early as 9 weeks after virus inoculation, the ability to mount either an IgM or IgG response to soluble protein antigens was lost. Neither B cell function, as assessed by lipopolysaccharide-induced blastogenesis or circulating B cell numbers, as assessed by immunofluorescence, differed between infected and control cats. These results suggest that FeLV-FAIDS infection may impair a subpopulation of T helper cells, that provides help for the production of antibody. Consistent with earlier observations of cats naturally infected with FeLV, our results indicate that early impairment of humoral immunity is an important component of the immunodeficiency syndrome induced by FeLV in cats.

Lymphocytic choriomeningitis virus-induced immunodepression: inherent defect of B and T lymphocytes

Infection of mice with lymphocytic choriomeningitis virus (LCMV) produces a rapidly induced immuno-suppression manifested by low lymphocyte proliferation in response to lipopolysaccharide (LPS) and concanavalin A (ConA). Analysis of the mechanisms underlying the unresponsiveness to these mitogens was undertaken at the cellular and molecular levels 7 days after infection. The selective elimination of CD8+ T cells and the results of coculture experiments demonstrated that unresponsiveness was not due to suppressor cells. Similarly, the role of inhibitory factors such as prostaglandins was excluded, since indomethacin, which inhibits their production, did not reverse the unresponsiveness. Analysis of different cytokines secreted by ConA-activated macrophages or T cells revealed that interleukin-1 (IL-1), synthesized during the T-dependent activation of macrophages by ConA, was normally produced by cells from LCMV-infected mice. In contrast, IL-2, which is produced by activated CD4+ T cells, was undetectable. Addition of exogenous IL-2 did not restore the proliferative response, although the p55-kilodalton protein of the IL-2 receptor was induced by ConA on CD4+ cells from LCMV-infected mice. Our results can be interpreted as showing that (i) unresponsiveness to mitogens of cells from LCMV-infected mice is not due to altered functions of the macrophages with respect to IL-1 production; (ii) CD4+ cells are activated, since the p55 chain of the IL-2 receptor is induced; (iii) the lack of IL-2 production cannot explain T-cell unresponsiveness, since addition of exogenous IL-2 did not restore the proliferative response. Taken together, these data suggest that T-lymphocyte unresponsiveness should be related to an inherent proliferative defect subsequent to T-cell activation and IL-2 receptor expression.

Feline leukemia virus infection as a potentiating cofactor for the primary and secondary stages of experimentally induced feline immunodeficiency virus infection

Preexistent feline leukemia virus (FeLV) infection greatly potentiated the severity of the transient primary and chronic secondary stages of feline immunodeficiency virus (FIV) infection. Of 10 FeLV-FIV carrier cats, 5 died of experimentally induced FIV infection, compared with 2 deaths in 10 cats infected only with FeLV and 1 death in 7 cats infected only with FIV. FIV-infected cats with preexistent FeLV infections developed severe depression, anorexia, fever, diarrhea, dehydration, weight loss, and leukopenia 4 to 6 weeks after infection and were moribund within 2 weeks of the onset of signs, whereas cats infected only with FIV developed much milder self-limiting gross and hematologic abnormalities. Pathologic findings in dually infected cats that died were similar to those observed previously in cats dying from uncomplicated primary FIV infection but were much more widespread and severe. Coinfection of asymptomatic FeLV carrier cats with FIV did not increase the levels of FeLV p27 antigen present in their blood over that seen in cats infected with FeLV alone. The amount of proviral FIV DNA was much higher, however, in dually infected cats than in cats infected only with FIV; there was a greater expression of FIV DNA in lymphoid tissues, where the genome was normally detected, and in nonlymphoid tissues, where FIV DNA was not usually found. Dually infedted cats that recovered from the primary stage of FIV infection remained more leukopenic than cats infected with FIV or FeLV alone, and their CD4+/CD8+ T-lymphocyte ratios were inverted. One of these cats developed what was considered to be an opportunistic infection. It was concluded, therefore, that a preexistent FeLV infection in some way enhanced the expression and spread of FIV in the body and increased the severity of both the resulting transient primary and chronic secondary stages of FIV infection. This study also demonstrated the usefulness of the FIV model in studying the role of incidental infectious diseases as cofactors for immunodeficiency-causing lentiviruses.

Effect of recombinant human interferon-alpha in vitro and in vivo on mitogen-induced lymphocyte blastogenesis in cats

The effect of recombinant human interferon-alpha (rHuIFN-alpha) in vitro and in vivo on mitogen-induced lymphocyte blastogenesis was evaluated in specific-pathogen-free cats. Pre-incubation of isolated feline peripheral blood lymphocytes (PBL) in vitro with either 10(4) or 10(3) International Units (U) of rHuIFN-alpha for 24 h significantly suppressed (P less than 0.001 and 0.01, respectively) blastogenic responses to the phytomitogens concanavalin A (Con A) and pokeweed mitogen (PWM). Lower doses of IFN (range, 10-10(-3) U/ml) neither suppressed nor enhanced mitogenesis. In the absence of phytomitogens, incubation of PBL with 10(4) - 10(2) U (P less than 0.001) or 10 U (P less than 0.05) of rHuIFN-alpha/ml resulted in a significant decrease in incorporation of [methyl-3H] thymidine into newly synthesized cellular DNA. Cultures of PBL exposed continuously for 4 days to rHuIFN-alpha doses of 10(4) U/ml or less did not demonstrate specific reductions in cell viability, indicating that the observed antiproliferative actions of IFN apparently were independent of any direct cytotoxic effects. To investigate the dose-response effects of rHuIFN-alpha in vivo on lymphocyte blastogenesis, individual groups of cats were evaluated on 3 consecutive days before and then 24 h after each cat was inoculated intramuscularly with either a high dose (10(6) U/kg), moderate dose (10(4) U/kg), or a relatively low dose (10(2) U/kg) of rHuIFN-alpha. Cats inoculated with 10(6) U of rHuIFN-alpha/kg had significantly reduced (P = 0.037) blastogenic responses to Con a at 24 h postinoculation compared to preinoculation values; mean PWM responses were also decreased, but this effect was not statistically significant. In contrast, inoculation of cats with either 10(4) or 10(2) U of rHuIFN-alpha/kg significantly enhanced (P = 0.05 or 0.008, respectively) Con A-induced blastogenesis and had no discernible effect on PWM responses. These findings suggest that very high doses of rHuIFN-alpha given parenterally may be associated with suppression of certain T-cell responses in cats; conversely, much lower doses may be immunoenhancing.

Evaluation of immunity to feline infectious peritonitis in cats with cutaneous viral-induced delayed hypersensitivity

Delayed-type hypersensitivity (DTH)-like reactions to feline infectious peritonitis (FIP) virus (FIPV) were induced in the skin of nine cats that were asymptomatic after a previous challenge-exposure with FIPV. Four of the nine previously challenge-exposed cats were negative for virus-neutralizing antibodies against FIPV at the time of intradermal (ID) testing for DTH. Two other cats tested for DTH when acutely ill with clinical FIP did not have cutaneous DTH responses to FIPV. Gross skin reactions to FIPV injected ID were observed in six of nine asymptomatic cats (67%) at postintradermal inoculation hours (PIH) 24, 48, and/or 72. The reactions consisted of focal, 1-5-mm to 2.5-cm diameter indurated or semi-firm, nonerythematous, slightly raised nodules. Microscopically, DTH-like reactions were observed in biopsies taken from the FIPV-inoculated skin of asymptomatic cats at PIH 24 to 72. The lesions consisted of perivascular and diffuse dermal infiltrations by macrophages, lymphocytes, and polymorphonuclear leukocytes (PMN). The dermal infiltrates, which were maximal at PIH 48 or 72, were predominantly mixed inflammatory cells (five of nine cats) or PMN (four of nine cats) at PIH 24, but later were predominantly mononuclear cells (six of nine cats) or mixed inflammatory cells (two of nine cats) at PIH 72. Five of nine cats (56%) with positive DTH skin responses had increased survival times after lethal ID challenge-exposure with FIPV compared to mean survival times in FIPV-naive, non-immune control cats that were DTH-negative when ID challenge-exposed. Four of nine DTH-positive cats (44%) resisted an ID challenge-exposure dose of FIPV that was fatal in both control cats, and two of the four remaining DTH-positive cats survived a third challenge-exposure with highly lethal doses of FIPV given intraperitoneally. Four of the six DTH-positive cats (67%) that died after re-challenge and were necropsied had lesions of noneffusive FIP, suggesting that cellular immunity may also be involved in the pathogenesis of noneffusive disease, whereas both control cats and both DTH-negative cats with clinical disease succumbed to effusive FIP. Seemingly, DTH responses to FIPV can be associated with an increased level of resistance to disease; however, this state of immunity is variable and apparently can be lost with time in some cats.

Clinical interpretation of leukocyte responses

Basic information has been presented concerning leukocyte (neutrophil, monocyte, lymphocyte, eosinophil, and basophil) function, production, kinetics, and response to various physiological and disease states. Using this information, veterinary practitioners should be able to interpret leukogram data from sick and healthy dogs and cats. Specifically, characteristic leukogram patterns such as physiological leukocytosis, corticosteroid-associated changes, and the presence of infection or severe inflammation should be recognized. In addition, interpretation of individual leukocyte responses should be possible. Several tables have been provided to assist in constructing a differential diagnosis to explain increases or decreases in absolute leukocyte numbers that lie outside of expected reference intervals.

Diseases associated with spontaneous feline leukemia virus (FeLV) infection in cats

More than 2000 cats sent for necropsy in order to provide a diagnosis were investigated immunohistologically using paraffin sections for the presence of a persistent infection with feline leukemia virus (FeLV). The spectrum of neoplastic and non-neoplastic diseases associated significantly with FeLV infection was determined statistically. Three-quarters of the cats with persistent FeLV infections died of non-neoplastic diseases and about 23% died of tumors, nearly exclusively those of the leukemia/lymphoma disease complex. A strong association with liver degeneration, icterus and a FeLV-associated enteritis was found in addition to the known association with non-neoplastic diseases and conditions such as anemia, bacterial secondary infections and respiratory tract inflammations due to the immunosuppressive effect of FeLV, hemorrhages and feline infectious peritonitis. Surprisingly, diseases and conditions like feline infectious panleukopenia, enteritis (of other types than FeLV-associated enteritis and feline infectious panleukopenia), glomerulonephritis, uremia and hemorrhagic cystitis were not associated with persistent FeLV infection. Another unexpected finding was that most pathogenic infectious agents demonstrated in the cats were not FeLV-associated either. Thus, immunosuppression due to FeLV infection seems to make the animals susceptible to certain pathogenic infectious agents, but not to the majority.

Genetic characterization of FLA, the cat major histocompatibility complex

The major histocompatibility complex (MHC) of the domestic cat (termed FLA) has been refractile to genetic and serological definition largely because of repeated failure to detect cytotoxic antibodies in multiparous cats or to elicit antibody following allogeneic lymphocyte immunization. We have developed a protocol for producing cytotoxic alloantisera in the cat following rejection of multiple surgical skin grafts. Of 59 cats subjected to grafting, 13 produced lymphocytotoxic antisera which had varying specificities among a panel of outbred cat cells. A population cluster analysis of the 13 alloantisera permitted the identification of six clusters of overlapping FLA specificities. Serological analysis of cells from 12 cat kindreds led to the definition of 24 allogeneic haplotypes, which segregate as a single Mendelian complex. Feline FLA antisera were characterized as class I or class II specific by immunoprecipitation of FLA gene products on lymphocyte cell surfaces. Abundant antigenic polymorphisms for both class I and class II MHC determinants were discovered, a result consistent with precedence in other species and the common expectation of the adaptive value of MHC variation. Development of feline MHC typing reagents and the definition of haplotypes for the cat hold promise for experimental analysis of valuable feline models for virus-induced immune deficiencies.

Antithymocyte globulin treatment of retrovirus-induced feline erythroid aplasia: in vivo and in vitro studies

The Kawakami-Theilen strain of feline leukemia virus (FeLV-KT) was used experimentally to produce erythroid aplasia in cats. The in vivo effects of goat anti-feline-thymocyte globulin (ATG) on hematopoiesis were investigated in FeLV-negative normal and FeLV-positive anemic cats. Treatment was initiated in anemic cats between 4 and 6 weeks postinoculation (PI) when erythroid progenitors were reduced to 10% of normal levels. During the first 2 weeks of treatment, ATG significantly increased the numbers of erythroid precursors in bone marrow from 15 to 35% in anemic cats and from 28 to 43% in normal cats. ATG stimulated a twofold increase of CFU-E and a threefold increase of CFU-GM in normal cats between 2 and 4 weeks after initiation of treatment but had no effect on CFU-E or CFU-GM in anemic cats. The in vivo effects of ATG were transient despite weekly treatment. Cats treated with normal globulin were not significantly different from untreated anemic control cats. In vitro treatment of low density bone marrow mononuclear cells with ATG plus complement increased CFU-E and BFU-E of bone marrow from cats prior to inoculation but not from viremic cats. These results indicate that, although ATG stimulates erythropoiesis and granulopoiesis in normal cats, it does not reverse retrovirus-induced erythroid aplasia.

Parameters of production and partial characterization of feline interleukin 2

The conditions for the production of feline interleukin 2 (IL-2) from peripheral blood leukocytes (PBL) and splenocytes by concanavalin A (Con A) stimulation are described. Feline IL-2 was quantitated by measuring DNA synthesis in the murine IL-2-dependent cell line, CTLL-20. In addition, feline IL-2 was generated for the maintenance of long-term cultures of Con A-stimulated feline PBL and for biochemical characterization. Finally, IL-2 production was evaluated from the PBL of feline leukemia virus (FeLV)-infected cats. Con A at 9.6 micrograms/ml produced a plateau of peak IL-2 activity from 24 to 48 h following stimulation. The tumor promoter, phorbol myristic acetate, stimulated feline IL-2 production and enhanced Con A-stimulated feline IL-2 production. Fetal calf serum (FCS) was not required for IL-2 production; however, FCS at 5% (v/v) allowed for maximal Con A-stimulated IL-2 production. Feline IL-2 generated from Con A-stimulated splenocytes migrated with an apparent molecular size of 13.7 to 23 kD by gel filtration chromatography and supported the proliferation of Con A-activated feline PBL at a final concentration of 0.3 to 0.9 units/ml.

Clinical and immunologic aspects of FeLV-induced immunosuppression

Cats exposed to the feline leukemia virus (FeLV) may mount an effective immune response and eliminate the virus, develop a non-viremic, latent infection or become persistently infected and shed the virus. Persistently infected cats commonly die of secondary opportunistic infections that result from FeLV-induced immunosuppression. The acquired immunosuppression is the most frequent and most devastating consequence of FeLV infection in the cat. Immunosuppression is targeted primarily to the cell-mediated immune system and has been attributed to the viral p15e envelope protein. The decreased IgG response and proliferative response to T cell mitogens is thought to be due to a defect in the helper cell function. As a result of T helper cell immunosuppression, infected cats may also have defective cytotoxic lymphocyte and activated macrophage functions which are regulated by their lymphokines. Research has shown that the virus causes a general suppression in the production of T cell-derived lymphokines, including gamma interferon and interleukin 2. A decrease in the function of polymorphonuclear leukocytes has also been reported and may contribute to deaths due to opportunistic infections in FeLV-positive cats. There are numerous parallels between the acquired immunodeficiency syndrome (AIDS) in man and the FeLV-induced immunodeficiency syndrome in cats. Frequent deaths due to opportunistic infections, lymphopenia, depressed cell-mediated immune responses to T cell-dependent antigens despite hypergammaglobulinemia and the presence of a long period of time between infection and the onset of clinical signs are just a few of the syndromes that are similar between the 2 retroviral diseases. A new strain of FeLV, FeLV-FAIDS has been associated with a naturally occurring immunosuppressive syndrome that is strikingly similar to AIDS in man. In addition, a T-lymphotropic retrovirus has recently been identified from cats with an immunodeficiency-like syndrome; this feline lentivirus disease is morphologically similar, but antigenically distinct from the human immunodeficiency virus, the cause of AIDS. Treatment for FeLV immunosuppression is primarily supportive. The development of a soluble tumor cell antigen vaccine has been shown to be efficacious in preventing FeLV infections.

Effects of increasing cyclic AMP or calcium on feline erythroid progenitors in vitro: normal cells are stimulated while cells from retrovirus-infected cats are suppressed

Pharmacologic modulators of cyclic 3',5'-adenosine monophosphate (cAMP) and calcium were added to cultures of bone marrow cells from normal cats and cats with retrovirus-induced erythroid aplasia (EA). Treatment with the following reagents increased the number of erythroid progenitors (CFU-e and BFU-e) in cells from normal cats: isoproterenol, dibutyryl cAMP, forskolin, RO-20-1724 and A23187. However, treatment of cells collected from viremic cats not only failed to enhance CFU-e and BFU-e but inhibited their growth. These studies suggest that EA is related to a non-reversible block of primitive erythroid progenitors or to direct inhibition of BFU-e and CFU-e growth.

Human immunodeficiency virus (HIV-1) cytotoxicity: perturbation of the cell membrane and depression of phospholipid synthesis

The molecular mechanism(s) by which human immunodeficiency virus (HIV-1) injures a T-cell line was studied. A pathological role for viral env proteins, which are inserted into the plasma membrane, has been previously demonstrated for HIV as well as other retroviruses which are cytopathic. We therefore initiated studies examining whether perturbations of the cell membrane or membrane-associated biochemical events may be occurring in cells acutely infected with HIV and whether such perturbations, if present, may be responsible for cytopathology. A human T-cell line (ERIC), which is sensitive to the cytopathic effects of HIVs, was infected with HTLV-IIIB and its membrane permeability to cations and its lipid metabolism were studied coincident with the peak expression of viral p24 and with the first sign of cytopathology (slowing of cell division) 72 to 96 hr after infection. It was found that the rate of influx of Ca2+ into the cell increased over that of uninfected cells and that phospholipid synthesis, primarily phosphatidylcholine, became depressed. Diacylglycerol, which serves both as an intermediate for synthesis of phospholipids and as a second-messenger for lymphocyte activation, was also greatly reduced. However, triglyceride synthesis was enhanced, indicating that not all lipid metabolic pathways were being shut down. This decreased membrane-synthetic ability and reduced second-messenger for cell division are likely to be important causes of HIV-1 cytopathology in ERIC cells. This hypothesis was supported by our finding that HIV cytopathology of ERIC cells could be partially prevented by treatment with compounds (diacylglyceride or PMA and transiently by oleic acid) which either replenish diacylglycerol in the infected cell and/or activate protein kinase C or phosphocholine cytidyltransferase, the latter being the rate-limiting step in synthesis of the major structural phospholipid in most cells.

Feline leukemia/sarcoma viruses and immunodeficiency

This chapter discusses the structure feline leukemia virus (FeLV) and pathogenesis of lymphomas and leukemias BY FeLV. FeLV is quite similar to the better-studied murine leukemia viruses in structure and genetic map. The virus particles bud from cytoplasmic membranes into either extracellular spaces or into vacuoles. FeLV has long been considered a typical noncytopathogenic, longlatency leukemia virus based on its behavior in fibroblasts in vitro . Recent evidence suggests that its in vivo behavior in critical target hemolymphatic tissues is as likely to be cytopathic as transforming. The type of FeLV-related disease that occurs and the disease-free interval probably are influenced by viral envelope proteins and glycoproteins and the consequences of proviral integration. FeLV subgroup specificity apparently determines when and what type of disease will occur. The ecotropic FeLV-A is the most frequent subgroup found in pet cats and is transmitted contagiously. Immunosuppression is the most frequent and the most devastating manifestation of FeLV viremia in clinical and experimental studies. It seems that multiple cell types and multiple processes are involved in the development of feline retrovirus-induced immunosuppression. Although no solid evidence is available for the malfunctioning of cat T helper cells because of the paucity of T-cell specific markers, the circumstantial evidence provided thus far indicates an impaired T helper function in FeLV-infected cats similar to that observed in humans infected with HIV. Studies on the pathogenesis of FeLV-induced immunosuppression might provide a valuable mode for a better understanding and means of control of human AIDS.

Feline leukemia virus: current status of the feline induced immune depression and immunoprevention

This is a review of the current knowledge of feline leukemia virus (FeLV) associated with immune depression observed in cats. It will focus on the clinical and experimental observations associated with feline retroviral infection and presence in vivo and in vitro. We will briefly describe retroviral-associated acquired immune deficiency syndrome associated with FeLV infection in the cat and specific cellular pathology associated with FeLV latency. In addition, we will focus on the action of FeLV-p15E in vitro and describe possible mechanisms of the FeLV-associated immunosuppression observed both in vivo and in vitro. Lastly, we will evaluate the current status of immunoprevention of FeLV. We will not attempt an in-depth analysis of the current literature; our focus is to review current findings as they relate to feline AIDS and immunotherapy.

Densitometric analysis of Western blot (immunoblot) assays for human immunodeficiency virus antibodies and correlation with clinical status

Western blot assays for antibodies directed against components of human immunodeficiency virus (HIV) associated with acquired immunodeficiency syndrome (AIDS) were examined with a densitometer and integrator. Antibody responses to seven HIV proteins were determined from the areas under the peaks of bands on blots from 430 seropositive individuals. Antibody responses corresponded qualitatively and quantitatively with clinical status. The Western blot assays examined were done on single specimens from individuals in one of four clinical states: asymptomatic with no risk factor identified, asymptomatic with risk factor(s) identified, AIDS-related complex, and AIDS. The ratios of gp41 antibody to p24 antibody and of gp41 antibody to total HIV antibodies increased, and the number of total HIV antibodies decreased progressively in these populations. Parameters were assigned to characterize the typical response found in AIDS: gp41 antibody/p24 antibody ratio, greater than or equal to 2.0; gp41 antibody/total HIV antibodies ratio, greater than or equal to 0.30; and number of total HIV antibodies, less than or equal to 25.0 signal units. Parameter match increased with progression of clinical status. These parameters were applied in a brief follow-up study of 34 HIV-infected asymptomatic individuals who developed AIDS-related complex or AIDS. Initial specimens showed a stronger correlation than our population data base had predicted, suggesting that the parameters have prognostic value. Densitometric analysis of antibody responses on Western blot assays of single or serial specimens should prove useful to physicians in staging and monitoring HIV-infected individuals and in predicting which individuals will progress to AIDS.

Saliva as a source of feline leukemia virus antigen for diagnosis of disease

An enzyme-linked immunosorbent assay (ELISA) for detection of feline leukemia virus (FeLV) p27 in saliva was tested for its accuracy and sensitivity in diagnosing FeLV infections. Saliva and serum samples from 564 clinical cases were tested with a 99.2% specificity. The overall accuracy of the saliva ELISA reactive to the serum ELISA was 97.9%. Experimentally, the ELISA saliva was the least sensitive in diagnosing early FeLV infections. However, the overall accuracy, ease of use, and simplicity of the test support its use as a screening procedure in clinical practice.

Sequence relationships of type D retroviruses which cause simian acquired immunodeficiency syndrome

Simian acquired immunodeficiency syndrome (SAIDS) in macaque monkeys is caused by type D retroviruses; three independent virus isolates are identified as SRV-1 (SAIDS retrovirus-serotype 1), SRV-2, and MPMV (Mason-Pfizer monkey virus). Virions from these three isolates have serologically related core antigens, but distinct surface proteins. Also, SRV-2 is unique since it apparently induces retroperitoneal fibromatosis in addition to SAIDS. The complete DNA sequence of molecularly cloned SRV-2 is presented and compared to the sequences of SRV-1 and MPMV and to the sequences of other retroviruses and retroviral-related elements in the genomes of eucaryotic cells. SRV-1 and MPMV show fewer than 6% differences in predicted amino acid sequences encoding gag, prt, pol, and the C-terminal env domain; SRV-2 displays about 15-18% differences in these regions when aligned with SRV-1 or MPMV. Greater variation of predicted amino acid sequences is noted in the externally located N-terminal env domains; SRV-1 and MPMV have 83% homology whereas SRV-2 has 58% homology with either SRV-1 or MPMV. Nucleotide sequences of the LTRs of SRV-1 and MPMV are 88% homologous; SRV-2 shows 70% homology with the LTRs of SRV-1 and MPMV. Comparisons of the predicted pol region amino acid sequences of these simian type D retroviruses with the pol gene of a type B retrovirus, mouse mammary tumor virus (MMTV), reveal about 50% homology. A human endogenous element related to the pol region of MMTV shows about 25% homology of amino acids with the pol sequences of SRV-1 or SRV-2. The prt genes of the simian type D retroviruses are similar in size and predicted amino acid sequence with the prt genes of MMTV and the hamster intracisternal type A particle genome. The C-terminal env domains of the avian type C retrovirus reticuloendotheliosis virus (REV) and the type C baboon endogenous virus (BaEV) have 60 and 85% predicted amino acid homology, respectively, with the C-terminal env domains of SRV-1, SRV-2, and MPMV. Within the gag and pol genes of the simian type D retroviruses there are striking homologies with the rat IgE-binding protein gene. Sequence relatedness of these type D retroviruses with type A, type B, and type C retrovirus genomes and with cellular sequences supports the notion that recombinational events contribute to the genesis and variation of retroviruses.(ABSTRACT TRUNCATED AT 400 WORDS)

Feline leukemia virus-associated enteritis--a condition with features of feline panleukopenia

Infection with feline leukemia virus (FeLV) was demonstrated immunohistologically in 218 necropsied cats suffering from enteritis. The animals were divided into three groups according to histopathological criteria. The first group exhibited the signs of feline panleukopenia in intestine, lymphoid tissues, and bone marrow. Only 1.6% of these animals were FeLV-infected. The animals of the second group had histopathological alterations as seen in cats suffering from feline panleukopenia, but these were found only in the intestine and not in lymphoid tissues or bone marrow. Of these 71.9% were infected with FeLV. The third group consisted of all other cats suffering from enteritis of which 6.3% were FeLV-positive. The association between FeLV infection and the lesions seen in the animals of group 1 (feline panleukopenia) and group 3 (other types of enteritis) is statistically not significant whereas the alterations exhibited by the cats of group 2 are significantly FeLV-associated. Cats with FeLV-associated enteritis (group 2) are of a mean age of about 2.5 years and are significantly older than animals with feline panleukopenia which are of a mean age of about half a year. Thus a FeLV-associated enteritis exists as a histopathologically recognizable condition which sometimes might be mistaken for feline panleukopenia in routine post-mortem investigations.

Virus-induced immunosuppression

Infection with a variety of viruses results in the suppression of the host's immune system. Several mechanisms thought to be responsible for this effect are discussed: infection and alteration of lymphocytes and macrophages, production of soluble suppressor factors, and the induction of suppressor cells. The clinical significance of virus-induced immunosuppression is also discussed.

Persistent viral infection. The carrier state

A persistent viral infection is one in which the virus in a replicating or non-replicating form persists in the host beyond the normal recovery and elimination period for that particular viral infection. The clinical significance and mechanisms of persistence, when known, are discussed for the important viral infections of dogs and cats. Particular emphasis is given to feline viral rhinotracheitis, feline calicivirus, canine distemper, and feline leukemia.

Post-exposure treatment with monoclonal antibodies in a retrovirus system: failure to protect cats against feline leukemia virus infection with virus neutralizing monoclonal antibodies

We have attempted to protect kittens against oronasal infection with FeLV (strain A/Glasgow-1) by treatment with a mixture of two virus-neutralizing (VN) MAbs (IgGl, K) directed against the same epitope on the viral glycoprotein gp70. Ten SPF 9-week-old kittens were infected on day 0 with 10(6) ffu FeLV and subsequently inoculated i.m. with MAbs every 2 days over a 20-day period at different times after infection. The results clearly show that no protection was achieved. It is unlikely that the amount of VN antibodies, the mode and route of their application or the infectious dose of FeLV used can account for the failure to protect cats against infection. Other possibilities which may explain the lack of protective effect are that the restricted epitope specificity of the MAb preparation used may have led to selection of neutralization-resistant virus mutants, or that other mechanisms than virus neutralization (complement-mediated lysis, antibody-dependent cell cytotoxicity), that may be involved in protection, function less efficiently with MAb. However, in the light of our finding that an early anti-idiotypic response is observed in all cats following administration of the MAb preparation, the rapid clearance of anti-FeLV MAb from the circulation is a more likely explanation. The data presented support our hypothesis that by administration of MAb-as compared to polyclonal antibody-a more vigorous anti-idiotypic response is elicited due to the presentation of only a limited set of idiotopes. This potential drawback of rapid clearance of MAbs as a consequence of an anti-idiotypic response might be overcome by the use of mixtures of MAbs resulting in a more heterogeneous set of idiotypic determinants.

Feline leukemia vaccine: efficacy, contents and probable mechanism

An effective subunit vaccine against feline leukemia virus infection and related diseases has been developed. The source of the vaccine immunogen is feline retrovirus persistently infected cells that continuously synthesize and shed virus polypeptides. Western blot analysis identifies FeLV-gp70, p27, p15, p12, and p10 in the subunit vaccine preparation. Cats immunized with the vaccine developed antibodies to a 70,000 MW protein of the vaccine that seems to be distinct from, but related to the FeLV-gp70 polypeptide.

Monocyte function in rhesus monkeys with simian acquired immune deficiency syndrome

Monocyte function in rhesus monkeys with simian acquired immune deficiency syndrome (SAIDS) was compared with that in age-matched normal juvenile rhesus monkeys. The functional tests were 1) chemotaxis, 2) phagocytosis of opsonized Candida albicans, 3) killing and/or growth inhibition of Candida albicans, 4) generation of respiratory burst, and 5) monocyte-derived macrophage response (morphology and/or respiratory burst) to stimulating agents such as lymphokines, gamma interferon, endotoxin, and phorbol myristate acetate. The monkeys tested had either clinical SAIDS (alive with lymphadenopathy, splenomegaly, and lymphopenia or neutropenia) or had terminal SAIDS (moribund due to the disease). Responses of monocytes from 14 monkeys with clinical SAIDS were indistinguishable from those of 9 normal juvenile rhesus monkeys, whereas monocytes from 3 monkeys with terminal SAIDS had enhanced phagocytosis and respiratory burst capacity. Chemotaxis, candidacidal/stasis activity, and response to stimulating agents were normal in these terminal cases. Plasma from the SAIDS monkeys was as capable of opsonizing yeasts and of being able to generate chemotactic factors by endotoxin as was control plasma. SAIDS retrovirus (SRV) was detected by co-cultivation of pure monocyte-derived macrophage cultures with Raji cells, an indicator cell line which forms syncytia in the presence of SRV. Four terminal SAIDS cases and one late-stage clinical SAIDS case were virus-positive when the number of macrophages in the cultures ranged from less than 50 to about 500. Terminal SAIDS monocyte-derived macrophages in culture as long as 17 days produced SRV. These data show that in monkeys with SAIDS the major effector functions of monocytes and macrophages involved in host defense are intact (even up until death). Additionally, some of the monocytes are productively infected, and these infected monocytes are viable and adherent in culture.

Reversal of feline retroviral suppression by indomethacin

The immunosuppressive effect of feline leukemia virus (FeLV) and its 15,000 dalton envelope protein (p15E) were studied to determine if the mechanism of action was due to an increase in prostaglandin production. We examined the effects of exogenous PGE1 and PGE2 on the normal Con A response of feline peripheral blood lymphocytes (PBL) and found them to be inhibitory. The addition of the cyclooxygenase inhibitor indomethacin to cells incubated with FeLV or FeLV p15E and Con A completely abrogated the viral suppressive effects. This reversal was titratable and time-dependent. Other non-steroidal anti-inflammatory (NSAI) drugs were found to have similar actions. Indomethacin was also able to increase the suppressed Con A response of PBL from FeLV-infected cats. Upon measurement of PGE2 levels from PBL cultured with FeLV, we found a decrease in PGE2 accumulation associated with FeLV presence during the first 24 h of culture. These findings indicate that FeLV does not cause its immunosuppressive effects by increasing PG production and suggests that indomethacin and the other tested NSAI drugs do not produce their effect by PG inhibition.

Immune-mediated disorders of cats

Immune-mediated disorders in cats share many clinical and pathologic similarities with their counterparts in other species. Cats, however, are unique among domestic animals owing to the involvement of feline leukemia virus. In addition, a number of other infectious organisms can produce immune-mediated sequelae--that is, FIP virus, FeSFV, and H. felis. Therefore, the diagnostic and therapeutic aims in the management of feline immune-mediated disorders must take into account the probability of a primary or underlying disease process.