Acyclovir and transmission of HIV-1 from persons infected with HIV-1 and HSV-2.

BACKGROUND: Most persons who are infected with human immunodeficiency virus type 1 (HIV-1) are also infected with herpes simplex virus type 2 (HSV-2), which is frequently reactivated and is associated with increased plasma and genital levels of HIV-1. Therapy to suppress HSV-2 reduces the frequency of reactivation of HSV-2 as well as HIV-1 levels, suggesting that suppression of HSV-2 may reduce the risk of transmission of HIV-1. METHODS: We conducted a randomized, placebo-controlled trial of suppressive therapy for HSV-2 (acyclovir at a dose of 400 mg orally twice daily) in couples in which only one of the partners was seropositive for HIV-1 (CD4 count, > or = 250 cells per cubic millimeter) and that partner was also infected with HSV-2 and was not taking antiretroviral therapy at the time of enrollment. The primary end point was transmission of HIV-1 to the partner who was not initially infected with HIV-1; linkage of transmissions was assessed by means of genetic sequencing of viruses. RESULTS: A total of 3408 couples were enrolled at 14 sites in Africa. Of the partners who were infected with HIV-1, 68% were women, and the baseline median CD4 count was 462 cells per cubic millimeter. Of 132 HIV-1 seroconversions that occurred after randomization (an incidence of 2.7 per 100 person-years), 84 were linked within couples by viral sequencing: 41 in the acyclovir group and 43 in the placebo group (hazard ratio with acyclovir, 0.92, 95% confidence interval [CI], 0.60 to 1.41; P=0.69). Suppression with acyclovir reduced the mean plasma concentration of HIV-1 by 0.25 log(10) copies per milliliter (95% CI, 0.22 to 0.29; P<0.001) and the occurrence of HSV-2-positive genital ulcers by 73% (risk ratio, 0.27; 95% CI, 0.20 to 0.36; P<0.001). A total of 92% of the partners infected with HIV-1 and 84% of the partners not infected with HIV-1 remained in the study for 24 months. The level of adherence to the dispensed study drug was 96%. No serious adverse events related to acyclovir were observed. CONCLUSIONS: Daily acyclovir therapy did not reduce the risk of transmission of HIV-1, despite a reduction in plasma HIV-1 RNA of 0.25 log(10) copies per milliliter and a 73% reduction in the occurrence of genital ulcers due to HSV-2. (ClinicalTrials.gov number, NCT00194519.)

Relationship between pre-existing viral reservoirs and the re-emergence of plasma viremia after discontinuation of highly active anti-retroviral therapy.

We examined the pathogenic significance of the latent viral reservoir in the resting CD4+ T cell compartment of HIV-1-infected individuals as well as its involvement in the rebound of plasma viremia after discontinuation of highly active anti-retroviral therapy (HAART). Using heteroduplex mobility and tracking assays, we show that the detectable pool of latently infected, resting CD4+ T cells does not account entirely for the early rebounding plasma HIV in infected individuals in whom HAART has been discontinued. In the majority of patients examined, the rebounding plasma virus was genetically distinct from both the cell-associated HIV RNA and the replication-competent virus within the detectable pool of latently infected, resting CD4 + T cells. These results indicate the existence of other persistent HIV reservoirs that could prompt rapid emergence of plasma viremia after cessation of HAART and underscore the necessity to develop therapies directed toward such populations of infected cells.

Cellular immune responses and viral diversity in individuals treated during acute and early HIV-1 infection.

Immune responses induced during the early stages of chronic viral infections are thought to influence disease outcome. Using HIV as a model, we examined virus-specific cytotoxic T lymphocytes (CTLs), T helper cells, and viral genetic diversity in relation to duration of infection and subsequent response to antiviral therapy. Individuals with acute HIV-1 infection treated before seroconversion had weaker CTL responses directed at fewer epitopes than persons who were treated after seroconversion. However, treatment-induced control of viremia was associated with the development of strong T helper cell responses in both groups. After 1 yr of antiviral treatment initiated in acute or early infection, all epitope-specific CTL responses persisted despite undetectable viral loads. The breadth and magnitude of CTL responses remained significantly less in treated acute infection than in treated chronic infection, but viral diversity was also significantly less with immediate therapy. We conclude that early treatment of acute HIV infection leads to a more narrowly directed CTL response, stronger T helper cell responses, and a less diverse virus population. Given the need for T helper cells to maintain effective CTL responses and the ability of virus diversification to accommodate immune escape, we hypothesize that early therapy of primary infection may be beneficial despite induction of less robust CTL responses. These data also provide rationale for therapeutic immunization aimed at broadening CTL responses in treated primary HIV infection.

Molecular cloning of a feline leukemia virus that induces fatal immunodeficiency disease in cats.

A replication-defective variant of feline leukemia virus was molecularly cloned directly from infected tissue and found to induce a rapid and fatal immunodeficiency syndrome in cats. Studies with cloned viruses also showed that subtle mutational changes would convert a minimally pathogenic virus into one that would induce an acute form of immunodeficiency. The data suggest that acutely pathogenic viruses may be selected against by current methods for isolation of the human and simian immunodeficiency viruses.

Genetic relationships determined by a DNA heteroduplex mobility assay: analysis of HIV-1 env genes.

The genetic diversity of human immunodeficiency virus (HIV) is a major concern thought to impact on immunologic escape and eventual vaccine efficacy. Here, simple and rapid methods are described for the detection and estimation of genetic divergence between HIV strains on the basis of the observation that DNA heteroduplexes formed between related sequences have a reduced mobility in polyacrylamide gels proportional to their degree of divergence. Reliable phylogenetic subtypes were assigned for HIV-1 strains from around the world. Relationships between viruses were closest when derived from the same or epidemiologically linked individuals. When derived from epidemiologically unlinked individuals, the relationships between viruses in a given geographic region correlated with the length of time HIV-1 had been detected in the population and the number of strains initiating widespread infection. Heteroduplex mobility analysis thus provides a tool to expedite epidemiological investigations by assisting in the classification of HIV and is readily applicable to the screening and characterization of other infectious agents and cellular genes.

Molecular epidemiology of HIV transmission in a dental practice.

Human immunodeficiency virus type 1 (HIV-1) transmission from infected patients to health-care workers has been well documented, but transmission from an infected health-care worker to a patient has not been reported. After identification of an acquired immunodeficiency syndrome (AIDS) patient who had no known risk factors for HIV infection but who had undergone an invasive procedure performed by a dentist with AIDS, six other patients of this dentist were found to be HIV-infected. Molecular biologic studies were conducted to complement the epidemiologic investigation. Portions of the HIV proviral envelope gene from each of the seven patients, the dentist, and 35 HIV-infected persons from the local geographic area were amplified by polymerase chain reaction and sequenced. Three separate comparative genetic analyses--genetic distance measurements, phylogenetic tree analysis, and amino acid signature pattern analysis--showed that the viruses from the dentist and five dental patients were closely related. These data, together with the epidemiologic investigation, indicated that these patients became infected with HIV while receiving care from a dentist with AIDS.

Antibodies to cell membrane antigens associated with human T-cell leukemia virus in patients with AIDS.

The acquired immune deficiency syndrome (AIDS), which has recently occurred at increasing rates in homosexual men, intravenous drug users, and others, is characterized by the development of Kaposi's sarcoma and several opportunistic infections including pneumonia caused by Pneumocystis carinii. Serum samples from patients with AIDS and from matched and unmatched control subjects were examined for the presence of antibodies to cell membrane antigens associated with human T-cell leukemia virus. Nineteen of 75 of the AIDS patients had antibodies directed to surface antigens of Hut 102, a reference T lymphoid cell line infected with leukemia virus, as did two of the 336 control subjects.

Antibodies to human T-cell leukemia virus membrane antigens (HTLV-MA) in hemophiliacs.

Along with homosexual men, Haitians, and intravenous drug abusers, hemophiliacs are at high risk of contracting acquired immunodeficiency syndrome (AIDS). An earlier study revealed that 36 percent of a group of the AIDS patients had antibodies to cell membrane antigens associated with the human T-cell leukemia virus (HTLV-MA), whereas only 1.2 percent of matched asymptomatic homosexual controls had these antibodies. In the present experiments, serum samples from 172 asymptomatic hemophiliacs were examined for the presence of antibodies to HTLV-MA. Such antibodies were detected in 5 to 19 percent of the hemophiliacs examined from four geographical locations, but in only 1 percent or less of laboratory workers, normal blood donors, donors on hemodialysis, or donors with chronic active hepatitis.

Genetic evaluation of suspected cases of transient HIV-1 infection of infants.

Detection of human immunodeficiency virus-type 1 (HIV-1) on only one or a few occasions in infants born to infected mothers has been interpreted to indicate that infection may be transient rather than persistent. Forty-two cases of suspected transient HIV-1 viremia among 1562 perinatally exposed seroreverting infants and one mother were reanalyzed. HIV-1 env sequences were not found in specimens from 20; in specimens from 6, somatic genetic analysis revealed that specimens were mistakenly attributed to an infant; and in specimens from 17, phylogenetic analysis failed to demonstrate the expected linkage between the infant's and the mother's virus. These findings argue that transient HIV-1 infection, if it exists, will only rarely be satisfactorily documented.

Evolutionary dynamics of HIV-1 and the control of AIDS.

Human immunodeficiency viruses (HIV) have exhibited an extraordinary capacity for genetic change, exploring new evolutionary space after each transmission to a new host. This presents a great challenge to the prevention and management of HIV-1 infection. At the same time, the relentless diversification of HIV-1, developing as it does under the constraints imposed by the human immune system and other selective forces, contains within it information useful for understanding HIV epidemiology and pathogenesis. Comparing the sheer mutational potential of HIV with actual data representing viral lineages that can survive selection suggests that HIV does not have unlimited capacity for change. Rather, clinical and bioinformatic data suggest that, even in the most diverse gene of the most highly variable organism, natural selection places severe limits on the portion of amino acid sequence space that ensures viability. This suggests some optimism for those attempting to identify sets of antigens that can generate effective humoral and cellular immune responses against HIV.

Position and degree of mismatches and the mobility of DNA heteroduplexes.

Heteroduplex mobility assay (HMA) is a fast and inexpensive method for determining relatedness between DNA sequences. Rapidly evolving viruses such as HIV-1 develop marked sequence differences in their genomes over the course of the epidemic and infection in a single individual. HMA can be used to monitor both processes. Here, we systematically evaluated the influence of single base mismatches on heteroduplex mobility. The impact of mismatches at nine different positions in 559 bp double-stranded DNA molecules, within a background of overall sequence divergence ranging from 1.97 to 9.65%, was evaluated in both non-denaturing and partially-denaturing acrylamide gels. We found that the electrophoretic mobility of heteroduplexes was proportional to the level of mismatch when that level exceeded 4.5%. Overall, mismatches near the center of the fragment and clustered mismatches tended to have an exaggerated influence on the mobility of heteroduplexes. Thus, the use of HMA for quantitative inference of genetic distances under the conditions we describe is of greatest utility at levels of mismatch >5%.

A study of vaccine-induced immune pressure on breakthrough infections in the Phambili phase 2b HIV-1 vaccine efficacy trial.

INTRODUCTION: The Merck Adenovirus-5 Gag/Pol/Nef HIV-1 subtype-B vaccine evaluated in predominately subtype B epidemic regions (Step Study), while not preventing infection, exerted vaccine-induced immune pressure on HIV-1 breakthrough infections. Here we investigated if the same vaccine exerted immune pressure when tested in the Phambili Phase 2b study in a subtype C epidemic. MATERIALS AND METHODS: A sieve analysis, which compares breakthrough viruses from placebo and vaccine arms, was performed on 277 near full-length genomes generated from 23 vaccine and 20 placebo recipients. Vaccine coverage was estimated by computing the percentage of 9-mers that were exact matches to the vaccine insert. RESULTS: There was significantly greater protein distances from the vaccine immunogen sequence in Gag (p=0.045) and Nef (p=0.021) in viruses infecting vaccine recipients compared to placebo recipients. Twenty-seven putative sites of vaccine-induced pressure were identified (p<0.05) in Gag (n=10), Pol (n=7) and Nef (n=10), although they did not remain significant after adjustment for multiple comparisons. We found the epitope sieve effect in Step was driven by HLA A∗02:01; an allele which was found in low frequency in Phambili participants compared to Step participants. Furthermore, the coverage of the vaccine against subtype C Phambili viruses was 31%, 46% and 14% for Gag, Pol and Nef, respectively, compared to subtype B Step virus coverage of 56%, 61% and 26%, respectively. DISCUSSION: This study presents evidence of sieve effects in Gag and Nef; however could not confirm effects on specific amino acid sites. We propose that this weaker signal of vaccine immune pressure detected in the Phambili study compared to the Step study may have been influenced by differences in host genetics (HLA allele frequency) and reduced impact of vaccine-induced immune responses due to mismatch between the viral subtype in the vaccine and infecting subtypes.

In vitro erythrocytopathic activity of an aplastic anemia-inducing feline retrovirus.

Although feline leukemia viruses (FeLV) cause a spectrum of proliferative and anti-proliferative diseases in vivo, in vitro studies demonstrating cell lineage-specific pathogenic properties of feline retroviruses have been rare. We describe here an efficient in vitro system that demonstrates the selective cytopathic effect of a molecularly cloned anemogenic FeLV (FeLV-Sarma-subgroup C; FSC) on erythroid progenitor cells. Forty-eight-hour coculture of normal feline bone marrow mononuclear cells with an underlayer of FSC-infected feline fibroblasts (FeF) resulted in infection of 60% to 90% of marrow mononuclear cells and pronounced depletion of early erythroid progenitor cells (BFUe). The dramatic depletion of BFUe was specific for FSC and did not occur in marrow cells infected with a molecularly cloned nonanemogenic subgroup A FeLV (FeLV 1161E; F6A). The ablation of BFUe by FSC in vitro paralleled both the decrease in BFUe and the induction of aplastic anemia in vivo. This combination of marrow cell infection by coculture and colony-forming unit (CFU) assessment by methylcellulose assay provides a reliable in vitro technique for studies of mechanisms involved in retrovirus-induced marrow aplasias.

Identification of an env G subtype and heterogeneity of HIV-1 strains in the Russian Federation and Belarus.

OBJECTIVE: To identify HIV-1 envelope sequence subtypes in infected individuals from the Russian Federation and Belarus. PATIENTS: A cohort of children infected after exposure to non-sterile needles during the 1988-1989 HIV-1 epidemic in southern Russia (n = 20) and HIV-1-seropositive individuals from Russia (n = 1) and Belarus (n = 7) infected via sexual transmission. METHODS: DNA samples derived from peripheral blood mononuclear cells were analysed for their HIV-1 genotypes by the heteroduplex mobility assay (HMA). The 1.3 kilobase-pair env gene fragments encoding a portion of gp120 were amplified by nested polymerase chain reaction, cloned and sequenced. The env sequences derived from these patients were aligned and phylogenetic neighbour-joining and maximum parsimony-derived trees generated. RESULTS: The env sequences derived from eight individuals infected in Russia and Belarus belong to subtype A (one), B (four), C (two), and D (one). Sequences derived from children, infected during parenteral manipulations in southern Russia, and one mother were closely related, but highly divergent, as a group, from all prototypic strains (genetic divergence, 17.2-22.9%). However, they clustered together with env sequences of the V1525 and LBV21-7 isolates from Gabon, recently described to be members of a new HIV-1 env subtype G. CONCLUSION: Extensive heterogeneity of HIV-1 subtypes was evident in the Russian Federation and Belarus. Our data also support the existence of an HIV-1 env genetic subtype G, and such isolates are now apparently present on both the African and European continents. These variants were identified through V3 peptide enzyme-linked immunosorbent assay screening and subsequent HMA analysis. The combination of these techniques represents a model for screening HIV variants within a large population.

Distribution of HIV-1 subtypes seen in an AIDS clinic in Sao Paulo City, Brazil.

OBJECTIVE: To determine the distribution of HIV-1 subtypes in Sao Paulo, Brazil. METHODS: Samples were obtained from 80 consecutive HIV-1-infected individuals attending the Immunodeficiency Clinic at the University of Sao Paulo in 1993. Peripheral blood mononuclear cells (PBMC) were separated by Ficoll-Hypaque gradient and a portion was used for routine CD4 counts; the remainder were frozen. PBMC were proteinase-K-digested and DNA-purified by organic extraction. Samples were amplified for the env region of HIV, and envelope sequence subtypes determined by heteroduplex mobility analysis using prototypic subtypes as references. A subset of these were also sequenced through the C2-V3 region of env. RESULTS: A total 69 of 80 samples yielded env polymerase chain reaction product enabling subtype determination; samples that did not amplify were those with low DNA yields. Among 12 injecting drug users (IDU) or sexual partners of IDU, four were typed as clade F and eight as clade B. Forty-three homosexual men or female sexual partners of bisexual men were typed as clade B. The 14 additional cases without known risk factors were typed as clade B. CONCLUSION: These data suggest that subtype F is related to injecting drug use in Brazil.

PIP: Serum samples from 80 consecutive HIV-1-infected individuals presenting to the Immunodeficiency Clinic at the University of Sao Paulo in 1993 were analyzed to determine the distribution of HIV-1 subtypes in the city. Peripheral blood mononuclear cells (PBMC) were separated using Ficoll-Hypaque gradient, a portion was used for routine CD4 counts, and the rest were frozen. PBMC were proteinase-K-digested and DNA-purified by organic extraction. The samples were amplified for the env region of HIV, and envelope sequence subtypes determined by heteroduplex mobility analysis using prototypic subtypes as references. A subset was also sequenced through the C2-V3 region of env. 69 samples yielded env polymerase chain reaction product enabling subtype determination. The samples which did not amplify had low DNA yields. Among 12 IV-drug users or their sex partners, four were typed as clade F and eight as clade B. 43 homosexual men or female sex partners of bisexual men were typed as clade B. The 14 additional cases with no known risk factor were typed as clade B.

Emerging genetic diversity of HIV-1 in South America.

OBJECTIVE: Genotype determination and risk group analysis of HIV-1 infected individuals in selected regions of South America. DESIGN: Cross-sectional convenience sampling of HIV-1-positive individuals in Peru, Ecuador, Uruguay and Paraguay from March, 1994 through September, 1998. METHODS: HIV-1-positive subjects were identified through the national AIDS surveillance program in each country. A standardized questionnaire was used to obtain demographic, clinical and risk factor data on each study subject. Viral DNA was extracted from participants' peripheral blood mononuclear cells either directly or after co-cultivation. A nested PCR was used to obtain selected fragments of the envelope genes for genotyping by the heteroduplex mobility assay (HMA). A 600 bp sequence encompassing the V3 loop was sequenced from a selection of 23 of these samples for phylogenetic analysis and confirmation of HMA genotype. RESULTS: Among the 257 successfully genotyped HIV-1-positive samples, genotype B was found in 98.3% (228/232) of those obtained from subjects in Peru, Ecuador, and Paraguay. In contrast, 56% (14/25) of the samples from Uruguay were genotype F, and the remainder were genotype B. Genotype F was detected for the first time in Peru (2/224) and Paraguay (1/4), and genotype A for the first time in Peru (1/224). Phylogenetic analysis confirmed the genotype identified by HMA in the 23 samples sequenced. There was no detectable genetic clustering of HIV-1 within the different high-risk groups or geographic locations. CONCLUSIONS: These findings verify and extend the presence of several different HIV-1 genotypes in South America.

Mammalian cell/vaccinia virus expression vectors with increased stability of retroviral sequences in Escherichia coli: production of feline immunodeficiency virus envelope protein.

Many eukaryotic DNA sequences, especially lenti-retrovirus proviral genomes and their env genes, are unstable when cloned in high-copy-number plasmids in Escherichia coli. Stability can be increased by the use of low-copy-number vectors, although plasmid yields are low. Vectors are described here that contain the intermediate-copy-number P15A ori for cloning, stable propagation and higher-yield production of plasmid DNA in E. coli, and the f1 ori for propagation as single-stranded phage. These vectors also have the capacity to direct high-yield production of protein in mammalian cells, and the option of incorporation into and expression via a T7 promoter in vaccinia virus. The SR alpha promoter, encephalomyocarditis (EMC) virus untranslated leader sequence, and poly(A) signal sequence serve as a high-yield mammalian cell expression cassette without the requirement for mRNA capping. A polyhistidine sequence is available at the 3' end of the cassette to facilitate chromatographic purification of protein. neo and gpt genes were included in some vectors to serve as selectable markers, and the dhfr gene was included in one to achieve gene amplification in mammalian cells. Dicistronic mRNAs can be generated by insertion of coding sequences up and downstream from the EMC leader. The utility of these vectors was shown through expression of feline immunodeficiency virus (FIV) Env protein, in conjunction with the tissue plasminogen activator (tPA) leader sequence.

Identification of low-level contamination of blood as basis for detection of human immunodeficiency virus (HIV) DNA in anti-HIV-negative specimens.

The significance of detection of human immunodeficiency virus (HIV) DNA by the polymerase chain reaction (PCR) in seronegative or seroconverting (SC) subjects remains controversial. In a previously reported study, we identified a case in which a specimen collected 12 months before seroconversion (pre-SC) was found repeatedly to be PCR positive in three experienced laboratories, while the 6-month pre-SC bleed was PCR-negative; PCR-based human leukocyte antigen (HLA)-DQA and -DRB typing of serial peripheral blood mononuclear cell (PBMC) samples from this case did not indicate a specimen mix-up or labeling error. To further investigate this case, we used HIV env sequence and DNA heteroduplex gel-shift analyses to characterize HIV quasispecies present in serial pre- and post-SC specimens. HIV env sequences and gel-shift pattern analyses from the 12-month pre-SC versus post-SC samples indicated that markedly distinct quasispecies were present, suggesting possible abortive infection followed by reinfection and subsequent seroconversion. However, the HIV burden of this pre-SC sample was very low (1 provirus/10(6) PBMCs), and the quasispecies was highly heterogeneous, findings suggesting long-term rather than recent HIV infection. To test the hypothesis that the index pre-SC sample was PCR positive owing to trace blood contamination during initial processing, we analyzed the three seropositive samples collected on the same date in 1985. One of these samples was highly related to the index pre-SC sample by env sequence and gel-shift methodologies.(ABSTRACT TRUNCATED AT 250 WORDS)

Disease progression and viral genome variants in experimental feline leukemia virus-induced immunodeficiency syndrome.

A fatal immunodeficiency syndrome with clinical and pathologic features similar to human AIDS is inducible in cats by experimental inoculation with a specific strain of feline leukemia virus (FeLV) called FeLV-FAIDS. The course of the feline disease is characterized by an age-dependent prodromal period during which a non-disease-specific, common form of proviral DNA is detected in bone marrow. Preceding clinical onset of immunodeficiency is production of high levels of specific, pathogenic variant genomes, primarily as unintegrated viral DNA, in bone marrow. Acute immunodeficiency syndrome (survival period approximately 3 months) is associated with a short prodromal period and appearance of a characteristic variant genome (variant A) that persists at high copy number as integrated and full-length unintegrated viral DNA in bone marrow. Chronic immunodeficiency syndrome (survival greater than 1 year) is marked by a longer prodromal period, a more gradual onset of severe clinical immunosuppression, and a predominance of other variant genomes that often contain substantial internal deletions. In both forms of the disease, tissue-specific replication of certain variant viruses is noted in the bone marrow, intestine, and lymph nodes. Evidence from in vitro and in vivo virus transmission studies indicates that the appearance of FeLV-FAIDS variant viruses reflects differential replication of viral genomes pre-existing in the inoculum rather than rapid de novo evolution of new variants within each animal. These results demonstrate that retrovirus-induced immunodeficiency disease in cats can be associated with and prefigured by the amplified replication of specific viral variants in target tissues.

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.