Selective thymocyte depletion and immunoglobulin coating in the thymus of cats infected with feline immunodeficiency virus

Magnetic resonance imaging of radiation-induced thymic atrophy as a model for pathologic changes in acute feline immunodeficiency virus infection

The development of a protocol to reproducibly induce thymic atrophy, as occurs in feline immunodeficiency virus (FIV) infection and other immunosuppressive diseases, and to consistently estimate thymic volume, provides a valuable tool in the search of innovative and novel therapeutic strategies. Magnetic resonance imaging (MRI) using the short tau inversion recovery (STIR) technique, with fat suppression properties, was determined to provide an optimized means of locating, defining, and quantitatively estimating thymus volume in young cats. Thymic atrophy was induced in four, 8-10-week-old kittens with a single, directed 500 cGy dose of 6 MV X-rays from a clinical linear accelerator, and sequential MR images of the cranial mediastinum were collected at 2, 7, 14, and 21 days post irradiation (PI). Irradiation induced a severe reduction in thymic volume, which was decreased, on average, to 47% that of normal, by 7 days PI. Histopathology confirmed marked, diffuse thymic atrophy, characterized by reduced thymic volume, decreased overall cellularity, increased apoptosis, histiocytosis, and reduced distinction of the corticomedullary junction, comparable to that seen in acute FIV infection. Beginning on day 7 PI, thymic volumes rebounded slightly and continued to increase over the following 14 days, regaining 3-35% of original volume. These findings demonstrate the feasibility and advantages of using this non-invasive, in vivo imaging technique to measure and evaluate changes in thymic volume in physiologic and experimental situations. All experimental protocols in this study were approved by the Institutional Animal Care and Use Committee (IACUC) at Auburn University.

Viral gene expression and provirus load of Orf-A defective FIV in lymphoid tissues and lymphocyte subpopulations of neonatal cats during acute and chronic infections

Neonatal cats were infected with a wild type (JSY3) or orf-A defective (JSY3DeltaORF-A) feline immunodeficiency virus (FIV) to determine the provirus load and level of viral gene expression at the acute versus chronic stages of infection. FIV DNA in the thymus, lymph node, peripheral blood mononuclear cells (PBMCs) and lymphocyte subpopulations at week 8 post-infection was lower in animals infected with JSY3DeltaORF-A as compared to that of JSY3. At week 16 we observed no significant difference in provirus load between the two groups except for B cells where it was higher in the JSY3 infection. In B cells proviral burden was found to be the same in animals infected with JSY3 for both time points. In the chronic stage, therefore, proviral burden dominates in B cells for JSY3, whereas the level of JSY3DeltaORF-A was lower with comparable values for all lymphocytes at both weeks 8 and 16. Gene expression profiles as measured by real time PCR for gag and rev transcripts revealed decreased levels of JSY3DeltaORF-A mRNAs as compared to that of JSY3. The JSY3 chronic phase infection showed viral gene expression to be higher in B cells relative to CD4+ and CD8+ cells. The presence of viral RNA in CD8 and B cells during the chronic infection implicates active virus replication. Hematological profiles revealed that there was a decline in the number of B cells in JSY3DeltaORF-A-infected cats during the chronic stage of infection while no significant change was observed in animals infected with the wild type virus. Comparative analysis of cell numbers to provirus load and levels of viral transcripts in CD4+ and CD8+, however, did not correlate cell numbers to the levels of viral DNA and gene expression. It remains to be determined whether the relatively high virus burden in B cells as compared to CD4+ and CD8+ cells reflects a role for Orf-A in a shift to B cell virus load during the chronic stage of FIV infection.

Immunopathogenesis of feline immunodeficiency virus infection in the fetal and neonatal cat

The global incidence of pediatric HIV infection is estimated at 2.3 million children, most acquiring the infection from their mothers in utero, peripartum, or postpartum. Pediatric HIV infection typically causes a rapidly progressive disease when compared with adult infection, due in part to the profound susceptibility of the neonatal thymus to productive infection or degenerative changes. Failed production of naive T-lymphocytes further limits the success of antiviral therapy to restore immunologic function. In this review, we explore the use of feline immunodeficiency virus (FIV) infection of domestic cats as an animal model for pediatric HIV infection. Cats infected with FIV represent the smallest host of a naturally occurring lentivirus, and the immunodeficiency syndrome elicited by FIV infection is similar to that of HIV-AIDS. The feline-FIV model uniquely reproduces several key aspects of immunosuppressive lentivirus infection of the thymus, allowing investigators to define viral determinants of pathogenicity, influence of host age on disease outcome, and therapeutic strategies to restore thymus function.

Assessment of CD4+ and CD8+ IFN-gamma producing cells by ELISPOT in naïve and FIV-infected cats

IFN-gamma is critical for the development of antiviral cell-mediated immunity in HIV infected humans and FIV infected cats. The ELISPOT has proven to be a technically straightforward assay to quantify the number of IFN-gamma producing cells and offers a reasonable alternative for the quantitative measurement of T-cell function in cats. We used a feline-specific ELISPOT to identify constitutive as well as Con A stimulated IFN-gamma production in T-cell subsets and determine if there were differences between purified (positively sorted) and negatively depleted populations from naïve and FIV infected cats. We found no difference in the total number of PBMC constitutively producing IFN-gamma in naïve and FIV+ cats. Con A exposure was associated with increased numbers of IFN-gamma producing PBMC in naïve, but not FIV+, cats. Equivalent numbers of CD4+ and CD8+ T cells constitutively expressed IFN-gamma in naïve cats. However, in FIV+ cats, the number of IFN-gamma producing CD8+ T-cells was approximately two-fold over that seen for CD4+ T-cells. We found minimal differences between purified (e.g. CD4+ or CD8+) and corresponding depleted (e.g. CD8- or CD4-) populations in samples from FIV+ cats. In contrast, depleted populations from naïve cats showed greater response to Con A than did purified populations. Thus, while determination of the number of IFN-gamma producing cells by feline-specific ELISPOT is a useful tool for the evaluation of the feline immune response, determination of the initial sample population and T-cell subset is critical for optimal interpretation of the IFN-gamma ELISPOT.

Immunohistochemical localization of feline immunodeficiency virus using native species antibodies

Feline immunodeficiency virus (FIV) is the feline analog of human immunodeficiency virus and a small animal model of human acquired immune deficiency syndrome (AIDS). We sought to identify early in vivo target cells in cats infected with clade B or C FIV. In tissues, however, neither mouse monoclonal nor rabbit polyclonal antibodies suitably detected FIV because of either insensitivity or lack of specificity. We therefore developed an immunohistochemical protocol using high-antibody-titer serum from cats chronically infected with FIV(Petaluma). Native species anti-FIV antibodies were labeled with biotinylated protein A before placement on tissues, and downstream signal was tyramide-amplified. This method revealed many productively infected cells in bone marrow, lymph node, thymus, mucosal-associated lymphoid tissue, and spleen, but few such cells in liver and none in kidney or brain. Concurrent labeling for virus and cell phenotype revealed that antigen-bearing populations were primarily T lymphocytes but included macrophages and dendritic cells. Our results demonstrate that FIV: 1) expands rapidly in T cells, 2) targets long-lived reservoir populations, and 3) is replicatively quiescent in brain at 3 weeks after infection. Use of native species antibodies for immunohistochemical detection of infectious antigens has application to other settings in which xenotypic (eg, mouse and rabbit) antibody sources are inadequate or unavailable.

Thymic lesions in cats infected with a pathogenic molecular clone or an ORF-A/2-deficient molecular clone of feline immunodeficiency virus

Previous studies using feline immunodeficiency virus (FIV) molecular clones lacking the putative transactivator gene (ORF-A/2) failed to address the issue of thymus pathogenesis or investigate the levels of viral replication in separate lymphoid compartments (Y. Inoshima, et al., J. Virol. 70:8518-8526, 1996; E. E. Sparger, et al., Virology 205:546-553, 1994). Using a highly pathogenic molecular clone of FIV, JSY3, and an ORF-A/2-deficient mutant, JSY3DeltaORF-A/2, we compared viral replication and the extent of thymic dysfunction as measured by the formation of lymphoid follicles and alteration of the thymocyte subsets. Viral replication was reduced in JSY3DeltaORF-A/2-infected cats as measured by lymphocyte coculture, immunohistochemistry, and quantitative PCR. Cell-associated viral load measured by lymphocyte coculture varied in a tissue-dependent manner with replication highest in lymphocytes isolated from the thymus, lower in those from the peripheral blood, and lowest in those from lymph node. Thymic proviral load and the number of viral p24 Gag-positive cells within the thymus detected by immunohistochemistry were also reduced. In addition, the onset of a reduced peripheral blood CD4/CD8 ratio was delayed in JSY3DeltaORF-A/2-infected cats. The formation and extent of thymic lymphoid follicular hyperplasia were similar in JSY3 and JSY3DeltaORF-A/2-infected cats as measured by anticytokeratin immunohistochemistry and flow cytometry for percent pan T-negative, immunoglobulin G-positive cells within the thymus. In contrast, comparison of thymocyte subpopulations demonstrated a reduced expansion of single-positive CD4(-) CD8(+) thymocytes in JSY3DeltaORF-A/2-infected cats. Level of viral replication, therefore, may not correlate with the formation of thymic lymphoid follicles but may correlate with the expansion of the single-positive CD4(-) CD8(+) thymocyte subpopulation.

CD8+ thymic lymphocytes express reduced levels of CD8beta and increased interferon gamma in cats perinatally infected with the JSY3 molecular clone of feline immunodeficiency virus

Biological isolates of feline immunodeficiency virus (FIV) cause a relative expansion of activated single-positive CD8(+) (SP CD8(+)) lymphocytes within the thymus of infected cats. In this study, thymic SP CD8(+) lymphocytes were analyzed from cats inoculated as neonates with a pathogenic molecular clone of FIV, JSY3, which was previously derived from the wild-type biological isolate FIV(NCSU-1) (NCSU-1). Four cats were inoculated intraperitoneally with NCSU-1 and compared with 11 cats inoculated with JSY3. Five control cats matched in litter and age were administered an intraperitoneal sham inoculum. Between 12 and 16 weeks postinoculation, interferon-gamma (IFN-gamma) mRNA was quantified by RT-PCR in freshly isolated thymocytes and peripheral blood mononuclear cells (PBMCs). The quantity of IFN-gamma mRNA was increased more than 10-fold in thymocytes and PBMCs of 13 of 13 FIV-inoculated cats as compared with the sham-inoculated controls. IFN-gamma mRNA coenriched with magnetically sorted CD8(+) PBMCs and single-positive (SP) CD8(+) thymocytes. Cells expressing IFN-gamma mRNA were located within the thymic perivascular zone, along the corticomedullary junction, and adjacent to lymphoid follicles. The expansion of thymic SP CD8(+) cells was associated with an increase in CD8alpha(+)/beta(neg) and CD8alpha(+)/beta(lo) phenotypes, the latter population resembling a previously reported memory/effector peripheral blood cell with FIV suppressor activity. From these data we conclude that JSY3 and NCSU-1 induce similar phenotypic changes in thymic and peripheral blood CD8(+) cells. Thus, JSY3 is pathogenic for the thymus in vivo and will be useful for defining determinants of the CD8(+) cell response in this pediatric AIDS model.

Antiviral therapy reduces viral burden but does not prevent thymic involution in young cats infected with feline immunodeficiency virus

The thymus is a major target organ in human immunodeficiency virus type 1 (HIV-1)-infected children and feline immunodeficiency virus (FIV)-infected young cats (G. A. Dean and N. C. Pedersen, J. Virol. 72:9436-9440, 1998; J. L. Heeney, Immunol. Today 16:515-520, 1995; S. M. Schnittman et al., Proc. Natl. Acad. Sci. USA 87:7727-7731, 1990; T. A. Seemayer et al., Hum. Pathol. 15:469-474, 1984; H.-J. Shuurn et al., Am. J. Pathol. 134:1329-1338, 1989; J. C. Woo et al., J. Virol. 71:8632-8641, 1997; J. C. Woo et al., AIDS Res. Hum. Retrovir. 15:1377-1388, 1999). It is likely that the accelerated disease process in children and cats is due to infection of the thymus during the time when generation of naive T lymphocytes is needed for development of the mature immune system. Zidovudine (ZDV) monotherapy, which is used to prevent and treat perinatal HIV-1 infection (R. Sperling, Infect. Dis. Obstet. Gynecol. 6:197-203, 1998), previously had been shown to reduce viral burden in FIV-infected young cats (K. A. Hayes et al., J. Acquir. Immune Defic. Syndr. 6:127-134, 1993). The purpose of this study was to evaluate the effect of drug-induced reduction of viral burden in the thymus on virus-mediated thymic involution and peripheral blood CD4 decline using FIV-infected cats as a model for pediatric HIV-1 infection. Eight-week-old cats were randomly assigned to uninfected, saline-treated; uninfected, ZDV-treated; FIV-infected, saline-treated; and FIV-infected, ZDV-treated groups. Parameters measured included blood lymphocyte numbers, viral load in blood and thymic tissue, and thymic histopathology. While the viral burden was significantly reduced by ZDV monotherapy in peripheral blood lymphocytes, plasma, and thymus, thymic lesions were similar for the treated and untreated FIV-infected cats. Further, markedly lowering the viral burden did not increase blood CD4 lymphocyte numbers or prevent their decline. The data suggest that an inflammatory process continued in spite of reduced virus replication. These observations imply that reducing virus load and limiting thymic inflammation are separate factors that must be addressed when considering therapeutic strategies aimed at preserving thymic function.

Relationship of lymphoid lesions to disease course in mucosal feline immunodeficiency virus type C infection

Feline immunodeficiency virus (FIV) infection typically has a prolonged and variable disease course in cats, which can limit its usefulness as a model for human immunodeficiency virus infection. A clade C FIV isolate (FIV-C) has been associated with high viral burdens and rapidly progressive disease in cats. FIV-C was transmissible via oral-nasal, vaginal, or rectal mucosal exposure, and infection resulted in one of three disease courses: rapid, conventional/slow, or regressive. The severity of the pathologic changes paralleled the disease course. Thymic depletion was an early lesion and was correlated with detection of FIV RNA in thymocytes by in situ hybridization. The major changes in thymic cell populations were depletion of p55+/S100+ dendritic cells, CD3- cells, CD4+/CD8- cells, and CD4+/CD8+ cells and increases in apoptosis, CD45R+ B cells, and lymphoid follicles. In contrast to thymic depletion, peripheral lymphoid tissues often were hyperplastic. Mucosally transmitted FIV-C is thymotropic and induces a spectrum of lymphoid lesions and disease mirroring that seen with the human and simian immunodeficiency virus infections.

Biphasic thymus response by kittens inoculated with feline immunodeficiency virus during fetal development

The objective of this study was to assess the response of the feline thymus to fetal infection with feline immunodeficiency virus (FIV), an animal model for human immunodeficiency virus infection. Thirteen feline embryos from four litters were directly inoculated with FIV during the sixth week postbreeding, a period corresponding to the late second trimester of pregnancy. Thymus tissue was collected and analyzed from randomly selected kittens at 2, 4, and 16 weeks postinoculation (PI) and compared to age-matched control kittens that did not receive fetal inoculations. Of three kittens evaluated at 2 weeks PI (week 8 of gestation), neither thymus:body weight ratio nor histologic structure differed from five age-matched control animals. However, analysis of thymocyte subpopulations by flow cytometry revealed a significant (P = 0.011) reduction in the percentage of cluster of differentiation (CD)4+/CD8+ cells from an average of 66% in control fetuses to 45% in infected fetuses. FIV RNA transcription, assessed by in situ hybridization using an FIVgag RNA probe, was widely distributed throughout the thymus in patterns suggestive of both stromal and parenchymal infection. By 4 weeks PI (week 1 postpartum), the thymus:body weight ratio was significantly reduced (P = 0.007) from 0.36% in five control kittens to 0.13% in four fetal inoculates. Severely atrophied thymus lobules supported minimal virus transcription and mean CD4+/CD8+ thymocyte percentages were lower (P = 0.021) in infected kittens (15%) compared to age-matched controls (66%). By 16 weeks PI (week 12 postpartum), thymus:body weight ratios of six inoculated kittens were not significantly different from six age-matched controls, suggesting that partial postnatal thymus regeneration had occurred. However, despite similar size, the regenerative thymus contained reduced percentages of CD4+/CD8+ thymocytes (infected: 40% versus control: 76%; P = 0.009) and increased percentages of CD4+/CD8- (11% versus 5%; P = 0.002) and CD4-/CD8+ (16% versus 9%; P = 0.035) lymphocytes. These changes were associated with widespread FIV transcription within thymic lymphocytes. Thus, the thymus of kittens infected with FIV during late fetal development is characterized by two distinct changes: neonatal atrophy and postnatal regeneration. Despite a recovery in thymus weight, thymus regeneration ineffectively restores the normal phenotypic distribution of thymocytes and supports FIV transcription.