Zidovudine in combination with alpha interferon and interleukin-2 as prophylactic therapy for FeLV-induced immunodeficiency syndrome (FeLV-FAIDS).

Zidovudine (3'-azido-3'-deoxythymidine; AZT) inhibited replication of an immunodeficiency-inducing strain of feline leukemia virus (FeLV-FAIDS) in vitro at concentrations of 0.5-0.005 micrograms/ml. A 25-30% additional antiviral effect was achieved in vitro when AZT was combined with human recombinant alpha interferon 2a (IFN alpha). Oral administration of AZT (20 mg/kg three times daily) to cats resulted in plasma concentrations of 3 micrograms/ml at 2 h post-administration with a T1/2 of approximately 1.60 h. Administration of AZT alone or in combination with IFN alpha or interleukin-2 (IL-2) throughout a 6-week treatment period enabled cats to resist challenge with FeLV-FAIDS. In contrast, those cats treated with IFN alpha or IL-2 alone became persistently antigenemic (core protein p27) in parallel with placebo-treated controls. Antigenemia remained undetectable in AZT-treated cats throughout an 80-day period post-inoculation (38 days after treatment was withdrawn). However, latent FeLV-FAIDS in bone marrow was detectable by in vitro culture of progenitor cells in the presence of hydrocortisone. Serial analysis of circulating p27 antigen, neutralizing antibody, and quantification of latent, reactivatable virus indicated that those animals receiving AZT in combination with IFN alpha were most able to resist FeLV-FAIDS challenge. This work provides additional evidence that early presymptomatic treatment employing combination chemoimmunotherapy can be effective in medical intervention of retroviral infection.

Protection against feline leukemia virus infection by use of an inactivated virus vaccine.

The protective immunity induced by 3 experimental FeLV vaccines were evaluated: Prototype inactivated FeLV vaccine developed from a molecularly cloned FeLV isolate (FeLV-FAIDS-61E-A); a mixture of immunodominant synthetic peptides corresponding to regions of the FeLV-Gardner-Arnstein-B (FeLV-GA-B) envelope proteins; and an adjuvant-disrupted but non-activated virus prepared from a non-cloned FeLV field isolate comprised of subgroup A and B viruses (FeLV-05821-AB). Included as controls were parallel groups of cats inoculated with adjuvants alone or with an established commercial FeLV vaccine. After each inoculation and after virulent virus challenge exposure, sera from all cats were assayed for ELISA-reactive antibody against purified FeLV, FeLV neutralizing (VN) antibody, and FeLV antigenemia/viremia--viral p27 antigen in serum and within circulating leukocytes. Immunity was challenged by oral/nasal exposure of vaccinated and control cats with FeLV-FAIDS-61E-A or FeLV-05821-AB, an infective, noncloned, tissue-origin, FeLV field isolate containing subgroup-A and -B viruses. Vaccine-induced immunity was assessed by comparing the postchallenge-exposure incidence of persistent viremia and the pre- and postchallenge exposure titers of VN and ELISA antibody in cats of the control and vaccine groups. The percentage of cats, that resisted development of persistent viremia after FeLV challenge exposure and the preventable fraction (PF) for the vaccine groups (which adjusts for the severity of the challenge and the degree of innate resistance in the controls) were as follows: adjuvant controls, 26%; FeLV-FAIDS-61E-A inactivated virus vaccine, 95% (PF = 93.2%); FeLV-GA-B peptide vaccine, 5% (-28.4%); FeLV-05821-AB noninactivated vaccine, 67% (55.4%); and commercial FeLV vaccine, 35% (12.2%). The prechallenge exposure mean VN antibody titer for each group was: less than 1:8 in the adjuvant controls; 1:43 in the FeLV-FAIDS-61E-A-vaccinated cats; less than 1:8 in the peptide-vaccinated cats; 1:38 in the noninactivated virus-vaccinated cats group; and 1:12 in the cats vaccinated with the commercial vaccine. Thus, induction of VN antibody in the vaccinated cats, although modest, appeared to be correlated with induction of protective immunity as defined by resistance to FeLV challenge exposure. Results of these studies indicate that inoculation of cats with an experimental inactivated virus vaccine prepared from a molecularly cloned FeLV isolate was most effective in stimulating protective immunity against heterologous and homologous FeLV challenge exposure.

Reversal of feline leukemia virus infection by adoptive transfer of activated T lymphocytes, interferon alpha, and zidovudine.

Previous experimental studies utilizing human recombinant interferon-alpha-2b (IFNalpha-2b) alone or with zidovudine (AZT) to treat established feline leukemia virus (FeLV) infection resulted in a significant reduction in circulating virus throughout a 49-day treatment period. However, the anti-FeLV effect of IFNalpha was limited by the production of IFNalpha-neutralizing antibodies detected 7 weeks after the start of treatment. AZT without IFNalpha had no effect on circulating virus load. To examine the hypothesis that combination chemoimmunotherapy might induce the clearance of FeLV infection, persistently infected cats were infused with activated lymphocytes, IFNalpha, and AZT 12 weeks after infection with FeLV. Recipient cats received weekly infusions of 1.46 x 10(8) lymphocytes activated in vitro with lectin/IL-2 comprised of 98% T cells and an even distribution of CD4+ and CD8+ lymphocytes. FeLV infection was cleared in 4 of 9 cats receiving combined therapy after four adoptive cell transfers. These cats remained negative for circulating virus during a 63-day treatment period (17 adoptive cell transfers) despite the production of anti-IFNalpha-neutralizing antibodies. Sequential development of virus-neutralizing and virus envelope antibody titers were detected in those cats which cleared retroviremia, an antiviral response that was absent in untreated control animals or nonresponders. Three of four responder cata remained negative for FeLV 95 days after treatment was discontinued. Treatment of cats with lymphocytes without chemotherapy failed to influence the course of FeLV infection. These results suggest that combined treatment using IFNalpha and adoptive lymphocyte transfer served to reconstitute antiviral humoral immunity, counteract immunosuppression, and induce the reversal of retroviremia.

Reversal of feline leukemia virus infection by adoptive transfer of lectin/interleukin-2-activated lymphocytes, interferon-alpha, and zidovudine.

Previous experimental studies utilizing human recombinant interferon-alpha-2b (IFN alpha-2b) alone or with zidovudine (AZT) to treat established feline leukemia virus (FeLV) infection resulted in a significant reduction in circulating virus throughout a 49-day treatment period. However, the anti-FeLV effect of IFN alpha was limited by the production of IFN alpha-neutralizing antibodies detected 7 weeks after the start of treatment. AZT without IFN alpha had no effect on circulating virus load. To examine the hypothesis that combination chemoimmunotherapy might induce the clearance of FeLV infection, persistently infected cats were infused with activated lymphocytes, IFN alpha, and AZT 12 weeks after infection with FeLV. Recipient cats received weekly infusions of 1.46 x 10(8) lymphocytes activated in vitro with lectin/IL-2 comprised of 98% T cells and an even distribution of CD4+ and CD8+ lymphocytes. FeLV infection was cleared in 4 of 9 cats receiving combined therapy after four adoptive cell transfers. These cats remained negative for circulating virus during a 63-day treatment period (17 adoptive cell transfers) despite the production of anti-IFN alpha-neutralizing antibodies. Sequential development of virus-neutralizing and virus envelope antibody titers were detected in those cats which cleared retroviremia, an antiviral response that was absent in untreated control animals or nonresponders. Three of four responder cats remained negative for FeLV 95 days after treatment was discontinued. Treatment of cats with lymphocytes without chemotherapy failed to influence the course of FeLV infection. These results suggest that combined treatment using IFN alpha and adoptive lymphocyte transfer served to reconstitute antiviral humoral immunity, counteract immunosuppression, and induce the reversal of retroviremia.

Longitudinal assessment of feline immunodeficiency virus kinetics in plasma by use of a quantitative competitive reverse transcriptase PCR.

Cats infected with feline immunodeficiency virus (FIV) develop a disease syndrome similar to that caused by human immunodeficiency virus type 1 (HIV-1) infection in humans. HIV-1 replication has been shown to correlate with the disease stage and progression. To assess replication kinetics and disease progression in early FIV infection, we developed a quantitative competitive reverse transcriptase PCR to measure the plasma virus load at serial time points after virus exposure. We found that an early peak viremia immediately preceded the onset of acute-phase symptoms in infected cats. Plasma virus levels remained high throughout the symptomatic phase of infection, which lasted for 8 to 10 weeks, and then declined as clinical symptoms resolved; however, all cats maintained significant plasma virus titers through 36 weeks postinfection. Early peak viral replication coincided with the initial precipitous decline in circulating CD4+ T lymphocytes. These results indicate that FIV kinetics are similar to those of HIV-1 during the acute and secondary phase of infection and that the plasma FIV load correlates with the disease stage. These results serve to further develop the FIV model and to enhance its usefulness for pathogenesis, vaccine development, and therapeutic studies related to HIV.

Plasma viral RNA load predicts disease progression in accelerated feline immunodeficiency virus infection.

Viral RNA load has been shown to indicate disease stage and predict the rapidity of disease progression in human immunodeficiency virus type 1 (HIV-1)-infected individuals. We had previously demonstrated that feline immunodeficiency virus (FIV) RNA levels in plasma correlate with disease stage in infected cats. Here we expand upon those observations by demonstrating that plasma virus load is 1 to 2 logs higher in cats with rapidly progressive FIV disease than in long-term survivors. Differences in plasma FIV RNA levels are evident by 1 to 2 weeks after infection and are consistent throughout infection. We also evaluated humoral immune responses in FIV-infected cats for correlation with survival times. Total anti-FIV antibody titers did not differ between cats with rapidly progressive FIV disease and long-term survivors. These findings indicate that virus replication plays an important role in FIV disease progression, as it does in HIV-1 disease progression. The parallels in virus loads and disease progressions between HIV-1 and FIV support the idea that the accelerated disease model is well suited for the study of therapeutic agents directed at reducing lentiviral replication.

Induction of accelerated feline immunodeficiency virus disease by acute-phase virus passage.

Development of feline immunodeficiency virus (FIV) infection in cats as a small animal model for lentiviral immunodeficiency disease has been hampered by the prolonged and variable disease course following experimental infection. To address this issue, we generated high-titer, unselected FIV stocks by pooling plasma from cats acutely infected with a subgroup C FIV isolate designated CABCpadyOOC (FIV-C-PGammer). Subsequent infection with this virus pool resulted in rapidly progressive, fatal disease in greater than 50% of infected cats. Accelerated FIV disease was characterized by rapid and progressive CD4+ T-cell loss, lymphadenopathy, weight loss, lymphoid depletion, and severe thymic atrophy. Mortality and rate of disease progression were affected by the age of each cat at infection and whether the virus source animal was in the acute or chronic stage of infection. The rapid FIV disease syndrome was consistently associated with systemic lymphoid depletion, clinical disease, and susceptibility to opportunistic infections, analogous to accelerated and/or terminal HIV-1 infection. The results of this study demonstrate that FIV infection is a valid small animal model for lentiviral immunodeficiency disease.

Alpha interferon (2b) in combination with zidovudine for the treatment of presymptomatic feline leukemia virus-induced immunodeficiency syndrome.

The therapeutic efficacies of human recombinant alpha interferon (IFN-alpha), IFN-alpha plus zidovudine (AZT), and AZT alone were evaluated in presymptomatic cats with established feline leukemia virus (FeLV)-acquired immunodeficiency syndrome (FAIDS) infection and high levels of persistent antigenemia. Subcutaneous injection of 1.6 x 10(6) U of human recombinant IFN-alpha 2b per kg delivered peak concentrations in plasma of 3,600 U/ml at 2 h postadministration with a half-life of elimination of 2.9 h. This dosage of IFN-alpha could be delivered to cats for up to 12 weeks without significant clinical toxicity. Oral administration of AZT (20 mg/kg three times daily) resulted in peak concentrations in plasma of 3 micrograms/ml at 2 h with a half-life of elimination of approximately 1.60 h. Treatment of FeLV-FAIDS-infected cats with IFN-alpha, either alone or in combination with orally administered AZT, resulted in significant decreases in circulating p27 core antigen beginning 2 weeks after the initiation of therapy. AZT alone had no effect on circulating virus antigen. Depending upon whether high (1.6 x 10(6) U/kg)- or low (1.6 x 10(4) to 1.6 x 10(5) U/kg)-dosage IFN-alpha was used, cats became refractory to therapy 3 or 7 weeks after the beginning of treatment. At these times, IFN-alpha-treated animals developed antibodies to IFN-alpha that were neutralizing, specific for human recombinant IFN-alpha, and dose dependent in magnitude. The results of this study indicate that human recombinant IFN-alpha is effective in reducing circulating virus antigenic load in cats persistently infected with FeLV-FAIDS. However, the continued efficacy of IFN-alpha therapy appeared to be limited by the formation of cytokine-specific neutralizing antibodies.