Cellular Immune Response of SIV‐Infected Rhesus Macaques

Statistical evaluation for detection of peptide specific interferon-gamma secreting T-cells induced by HIV vaccine determined by ELISPOT assay

Interferon-gamma (IFN-gamma) secreting T-cells play an important role in immunodeficiency virus (HIV) disease pathogenesis. A common technique to determine the efficacy of the HIV vaccines in murine models is to compare the number of IFN-gamma secreting T-cells induced by HIV vaccine to a control group. The measurement of IFN-gamma secreting T-cells relies on an ELISPOT assay. This assay is carried out in triplicate wells from the same mouse on the same ELISPOT plate. The numbers of spot forming cells (SFC) from these wells are correlated counts. Traditionally, simple statistical methods, such as ANOVA or Kruskall-Wallis tests, are performed on means by mouse. These approaches ignore the fact that the data are correlated counts. Count data are usually assumed to follow the Poisson distribution. However, some count data exhibit overdispersion that can affect the test statistics. The negative binomial distribution is an alternative to the Poisson distribution in the presence of overdispersion. Hence, negative binomial regression is a more suitable approach for overdispersed count data. In this study, we used a negative binomial regression to determine that IL-12 was a good adjuvant. The results of the study using a negative binomial regression are discussed.

Use of interleukin 15 to enhance interferon-gamma production by antigen-specific stimulated lymphocytes from rhesus macaques

The enzyme-linked immune spot (ELISPOT) assay is receiving increased attention as a means for quantifying antigen-specific CD8 T-cell responses in rhesus macaques. Further improving the sensitivity of this assay could aid in the evaluation of vaccine candidates and/or immune therapeutic candidates. Interleukin (IL)-15 has been demonstrated to stimulate expansion of human immunodeficiency virus (HIV)-specific cytotoxic T lymphocytes (CTL) and to regulate homeostatic proliferation of CD8+ memory cells. We evaluated the in vitro effect of IL-15 to increase the detection of interferon-gamma (IFN-gamma) production by antigen-specific stimulated lymphocytes from a group of rhesus macaques exposed to simian-human immunodeficiency virus (SHIV) and a second group infected with SIVmac251, before and after antiretroviral treatment (ART). Results from these studies demonstrate that the presence of IL-15 during stimulation in a peptide-based ELISPOT assay greatly enhanced IFN-gamma production in both SHIV and simian immunodeficiency virus (SIV)-infected macaques. IFN-gamma production was mainly mediated by CD8 lymphocytes. The optimal concentrations of IL-15 that give enhancement of IFN-gamma production to specific antigen, without a significant increase in the spontaneous IFN-gamma release, ranged from 0.5 to 2.5 ng/ml. The mean number of IFN-gamma spots was increased 3.1- to 3.6-fold in response to SIV gag or HIV env peptide pools, respectively, in peripheral blood mononuclear cells (PBMC) from SHIV-infected macaques. Similarly, in SIV-infected macaques, IL-15 increased the mean number of IFN-gamma spots 2.7-fold in response to both SIV gag and env peptide pools. In samples obtained after ART in the same macaques, the increase factor was 2.5 for SIV gag and 1.8 for the env peptide pools. Thus, the sensitivity of the ELISPOT assay can be enhanced by addition of IL-15. This modified assay will be useful for detection of low frequencies of IFN-gamma producing cells in rhesus macaques.

ELISPOT assay for detection of peptide specific interferon-gamma secreting cells in rhesus macaques

A reliable procedure to measure antigen specific T cell responses in rhesus macaques is required to determine the efficacy of vaccines and immunotherapies. The currently available T cell assays are poorly quantifiable or technically difficult to perform. Classical 51Cr-release cytotoxic T cell (CTL) assays are cumbersome and difficult to quantitate reproducibly. Detection of specific T-cell using MHC-peptide tetrameric complexes is highly sensitive, but requires knowledge of MHC type and prior identification of T cell epitopes. We therefore developed a rhesus interferon-gamma (IFN-gamma) ELISPOT assay capable of detecting IFN-gamma secretion in response to stimulation with pooled 20-mer peptides. Peripheral blood mononuclear cells (PBMCs) from rhesus monkeys immunized with a DNA vaccine and recombinant canary pox encoding the Plasmodium knowlesi circumsporozoite protein (PkCSP) were incubated with pools of peptides from PkCSP. Positive responses to peptide pools and individual peptides ranging from 100 to 450 spot forming cells (SFC)/10(6) PBMC were detected in four of four immunized monkeys and in zero of two control monkeys. In two monkeys studied in detail, the IFN-gamma response was focussed on a single 20-mer peptide, QGDGANAGQPQAQGDGANAG, and was dependent on CD4(+), but not CD8(+), T cells. Background responses in control monkeys and preimmunization PBMCs ranged from 10 to 50 SFC/10(6) PBMC. The average within assay and between assay coefficients of variation (CV) for this peptide ELISPOT were 21.9 and 24.7%, respectively. This peptide IFN-gamma assay will be a useful tool for evaluation of T cell responses in rhesus macaques.

Induction of antigen-specific killer T lymphocyte responses using subunit SIVmac251 gag and env vaccines containing QS-21 saponin adjuvant

Subunit vaccines based on recombinant proteins have proved useful for inducing antibody responses and they are safe for widespread use because they do not contain any live components. Unfortunately, they do not typically induce the types of cell-mediated immune responses required to control viral pathogens; specifically, they do not induce CD8+ cytotoxic T lymphocyte (CTL) responses. To increase the immunogenicity of recombinant proteins, we have used the QS-21 saponin adjuvant in subunit vaccine formulations. In the current study, experimental subunit vaccine formulations containing recombinant p55gag or gp120env proteins from the mac251 strain of the simian immunodeficiency virus (SIVmac251) and the QS-21 adjuvant were used to immunize rhesus macaques. These formulations induced SIV gag- or env-specific cellular immunity that was detectable in vitro and included killer cell activity. The induction of killer cells required prior vaccination and the responses were antigen specific for the immunogens contained in the vaccine formulations. Autologous target cells were required to detect these responses, suggesting genetic restriction, and effector cells appeared to be present in both the CD4+ and CD8+ T lymphocyte subpopulations. These data suggest that the vaccine-induced killer cell activity that was detected was mediated by both CD4+ and CD8+ lymphocytes. Despite the presence of these killer cells, all of the animals became infected with the SIVmac251 on experimental challenge. These findings demonstrated that antigen-specific killer cell responses could be induced by a subunit vaccine formulated with the QS-21 saponin adjuvant. The characteristics of the responses suggested that the effector cells were T lymphocytes, expressing either CD4 or CD8.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of cytotoxic T lymphocyte responses to SIV proteins in SIV-infected macaques using antigen-specific stimulation with recombinant vaccinia and fowl poxviruses

Methods to analyze CD8+ CTL responses to simian immunodeficiency virus (SIV)-encoded proteins are essential to understand lentivirus immunopathogenesis and protective immune responses. Recombinant infectious shuttle vectors are useful for analyzing CTL responses to many viruses, including HIV. Therefore, CTL responses in SIV-infected Macaca fascicularis to SIV env and SIV gag/pol were evaluated using specific antigen stimulation with recombinant vaccinia (rVV) and fowl poxviruses (rFPV) containing SIV genes. Peripheral blood mononuclear cells from SIV-infected animals were stimulated with autologous cells infected with rVV expressing SIV env/gag/pol, and CTLs specific for SIV env and for SIV gag/pol were detected by testing for lytic activity in target cells expressing these genes separately. Lymphocyte subset purifications from the effector population demonstrated that the CTL response was mediated by CD8+ cells, and the use of brefeldin A to selectively block antigen presentation in association with MHC class I products affirmed this cytolytic activity was class I restricted. The use of rVV to analyze responses to SIV genes is potentially problematic in hosts immunized to vaccinia. Fowl poxvirus is an alternative virus that has many of the molecular advantages of vaccinia virus but is genomically distinct. Therefore, the ability of rFPV to expand and detect SIV-specific CTLs was evaluated. Although there was no cytopathic effect following infection with rFPV, macaque cells infected with this vector did express rFPV gene products, and could be used as stimulator and target cells to detect SIV-specific CD8+ CTLs. The results suggest that these recombinant viral vectors can be used to specifically stimulate CD8+, MHC class I-restricted CTLs reactive to SIV proteins, and should facilitate evaluating CTL responses in both SIV-infected animals and animals vaccinated against SIV.

Use of particulate forms of protein antigens to increase the sensitivity of antigen-specific proliferation assays

Different forms of recombinant HIV-1 gp160 and tetanus toxoid were adsorbed onto latex microspheres and this particulate form of the proteins was used to measure antigen-specific proliferation in vaccinated rhesus macaques. Proliferative responses to proteins bound to microspheres were significantly greater and allowed for the detection of antigen-specific responses that were not detected using soluble proteins. The responses were antigen-specific and required prior immunization of the animals. Additionally, the presence and magnitude of the proliferative responses was associated with antibody responses to the same proteins suggesting the results were representative of in vivo responses and that the assay format did not induce in vitro artifacts.

Induction of feline immunodeficiency virus-specific cytolytic T-cell responses from experimentally infected cats

We have examined the in vitro induction and activity of feline immunodeficiency virus (FIV)-specific cytolytic T cells obtained from cats experimentally infected for 7 to 17 weeks or 20 to 22 months with the Petaluma isolate of FIV. Normal or FIV-infected autologous and allogeneic T lymphoblastoid cells were used as target cells in chromium-51 or indium-111 release assays. When effector cells consisted of either fresh peripheral blood mononuclear cells or concanavalin A- and interleukin-2-stimulated cells, only low levels of cytotoxicity were observed. However, the levels of FIV-specific cytotoxicity were consistently higher in both groups of cats following in vitro stimulation of the effector cells with irradiated, FIV-infected autologous T lymphoblastoid cells and interleukin-2. The effector cells lysed autologous but not allogeneic FIV-infected target cells and were composed predominantly of CD8+ T cells, indicating that the FIV-specific cytotoxicity measured in this system is mediated by CD8+, major histocompatibility complex class I-restricted T cells. These studies show that FIV-specific cytolytic T cells can be detected as early as 7 to 9 weeks postinfection, and they define a system to identify virus-encoded epitopes important in the induction of protective immunity against lentiviruses.