William R. Green

Antibody to the ligand of CD40, gp39, blocks the occurrence of the acute and chronic forms of graft-vs-host disease.

Chronic and acute graft-versus-host disease (cGVHD and aGVHD) result from donor cells responding to host disparate MHC alleles. In cGVHD (H-2d-->H-2bd), heightened polyclonal immunoglobulin production is due to the interaction of donor allospecific helper T cells (Th) and the host B cells. In vivo administration of antibody to the ligand for CD40, gp39, blocked cGVHD-induced serum anti-DNA autoantibodies, IgE production, spontaneous immunoglobulin production in vitro, and associated splenomegaly. Antibody production remained inhibited for extended periods of time after termination of anti-gp39 administration. Antiallogeneic CTL responses induced in a GVHD were also prevented by the in vivo administration of anti-gp39 as was the associated splenomegaly. These data suggest that CD40-gp39 interactions are critical in GVHD and that CD40-gp39 may be a valuable ligand-receptor pair for targeting immunotherapeutic agents to control GVHD.

In the absence of a CD40 signal, B cells are tolerogenic

When B cells are deprived of signaling through CD40, they exhibit the ability to induce T cell tolerance. The in vivo administration of anti-gp39 and allogeneic B cells diminished the ability of mice to mount an allogeneic response. Tolerance induction was specific for the haplotype expressed on the allogeneic B cells. Selective allospecific unresponsiveness was induced in the CD8 and CD4 compartments by the administration of anti-gp39 and class II-deficient B cells or class I-deficient B cells, respectively. As predicted by studies with anti-gp39 treatment, diminished allospecific responsiveness was induced by the administration of B cells to mice genetically deficient in gp39. Taken together, these data are consistent with the premise that deprivation of CD40 signaling engenders B cells with enhanced tolerogenicity. These studies provide insights into the tolerogenic capacity of resting B cells and outlines a practical approach to exploit this function.

Non-traditionally derived CTL epitopes: exceptions that prove the rules?

Abstract Recent data indicate that cytotoxic T lymphocyte (CTL) epitopes in viral, tumor and model systems are encoded by non-primary open reading frame (ORF) sequences, as well as other non-traditional sources. Here, Shawn-Marie Mayrand and William Green review these findings and discuss whether these non-traditionally derived CTL epitopes influence the shaping of the T-cell repertoire and immune regulation.

Alternative Translational Products and Cryptic T Cell Epitopes: Expecting the Unexpected

Although CD8 T cell epitopes have been studied extensively, often overlooked are unconventional cryptic epitopes generated from nontraditional sources of peptides/proteins and/or mechanisms of translation. In this review, we discuss alternative reading frame epitopes, both mechanistically and also in terms of their physiologic importance in the induction of antiviral and antitumor CTL responses. Issues of the influence of cryptic translational products on foreign and self-Ag diversity, thymic selection, and the T cell repertoire; disease pathogenesis; and approaches to vaccine design are discussed in context of the potentially large impact of unconventional epitopes on T cell immunity.

Myeloid-Derived Suppressor Cells in Murine Retrovirus-Induced AIDS Inhibit T- and B-Cell Responses In Vitro That Are Used To Define the Immunodeficiency

ABSTRACT Myeloid-derived suppressor cells (MDSCs) have been characterized in several disease settings, especially in many tumor systems. Compared to their involvement in tumor microenvironments, however, MDSCs have been less well studied in their responses to infectious disease processes, in particular to retroviruses that induce immunodeficiency. Here, we demonstrate for the first time the development of a highly immunosuppressive MDSC population that is dependent on infection by the LP-BM5 retrovirus, which causes murine acquired immunodeficiency. These MDSCs express a cell surface marker signature (CD11b+ Gr-1+ Ly6C+) characteristic of monocyte-type MDSCs. Such MDSCs profoundly inhibit immune responsiveness by a cell dose- and substantially inducible nitric oxide synthase (iNOS)-dependent mechanism that is independent of arginase activity, PD-1–PD-L1 expression, and interleukin 10 (IL-10) production. These MDSCs display levels of immunosuppressive function in parallel with the extent of disease in LP-BM5-infected wild-type (w.t.) versus knockout mouse strains that are differentially susceptible to pathogenesis. These MDSCs suppressed not only T-cell but also B-cell responses, which are an understudied target for MDSC inhibition. The MDSC immunosuppression of B-cell responses was confirmed by the use of purified B responder cells, multiple B-cell stimuli, and independent assays measuring B-cell expansion. Retroviral load measurements indicated that the suppressive Ly6Glow/± Ly6C+ CD11b+-enriched MDSC subset was positive for LP-BM5, albeit at a significantly lower level than that of nonfractionated splenocytes from LP-BM5-infected mice. These results, including the strong direct MDSC inhibition of B-cell responsiveness, are novel for murine retrovirus-induced immunosuppression and, as this broadly suppressive function mirrors that of the LP-BM5-induced disease syndrome, support a possible pathogenic effector role for these retrovirus-induced MDSCs.

Cytolytic CD8 + T Cells Directed against a Cryptic Epitope Derived from a Retroviral Alternative Reading Frame Confer Disease Protection

Cytolytic CD8+ T cells (CTL) are key to the immune response that controls virus infections and mediates disease protection. The ability of CTL to induce apoptosis of infected cells and/or limit viral replication is determined by recognition of processed viral peptide epitopes on the surface of the target cell. An understudied source of MHC class I-presented peptides is the aptly named “cryptic epitopes,” defined by their nontraditional methods of generation, including derivation from alternative reading frames (ARFs). Although ARF-encoded epitopes have now been documented in a few systems, their potential functional relevance in vivo has been debated. In this study, we demonstrate the physiological significance of an ARF-derived CTL epitope in a retrovirus-induced disease model. We show that disease-susceptible CD8-deficient mice reconstituted with CTL specific for the retroviral ARF-derived SYNTGRFPPL epitope controlled an infection by the LP-BM5 retrovirus isolate, evidently at the level of viral clearance, resulting in protection of these mice from disease. These data indicate that ARF-derived epitopes are indeed relevant inducers of the immune system and demonstrate the importance of atypically generated peptides as functional Ag with a physiologic role in disease protection.

The influence of natural antibody specificity on antigen immunogenicity

The natural antibody repertoire in humans, apes and Old World primates is distinct from the repertoire of all other placental mammals, and encodes antibodies specific for the carbohydrate epitope Galα1–3Galβ1–4GlcNAc‐R (αGal). Here, we examined whether conjugating antigens to the αGal epitope can augment their immunogenicity in α(1,3)galactosyltransferase knockout mice (GT0 mice) which, like humans, produce αGal‐specific antibodies. Immunization of GT0 mice with BSA conjugated to αGal (αGal‐BSA) led to significant production of anti‐BSA IgG antibodies without the need for adjuvant. This response was dependent on the presence of αGal‐reactive antibodies. Immunization of wild‐type mice with αGal‐BSA failed to induce an anti‐BSA response. The presence of αGal‐reactive antibodies also led to an increase in the T cell response to BSA following immunization with αGal‐BSA when compared with mice that received BSA alone, resulting in an increased frequency of IFN‐γ− and IL‐4‐producing BSA‐specific T cells. In addition, the ability to produce αGal‐reactive antibodies enhanced the cytotoxic T lymphocyte anti‐viral antigen response following vaccination with murine leukemia virus transformed cell lines that express αGal on their cell surface. Natural antibodies that bind αGal therefore play a key role in increasing the efficiency of priming to antigens decorated with αGal epitopes.

Quantitative analysis of LP-BM5 murine leukemia retrovirus RNA using real-time RT-PCR

Murine AIDS (MAIDS) develops in susceptible mouse strains after infection with the LP-BM5 murine leukemia virus (MuLV) complex that contains a mixture of defective (BM5def) and replication-competent viruses. While the BM5def virus is the causative agent in MAIDS, the replication-competent viruses in LP-BM5, including ecotropic MuLV (BM5eco), are required for BM5def propagation and thus function as helper viruses. We describe quantitative real-time RT-PCR assays for RNA encoded by the BM5def and BM5eco components of LP-BM5. The assays were used to standardize better the input doses of LP-BM5 viruses across viral preparations and to quantify BM5def and BM5eco gag RNA levels in spleen and blood cells from MAIDS-susceptible and -insusceptible infected mice. Spleens of MAIDS-susceptible infected mice harbored approximately similar levels of BM5def gag RNA as infected spleens of mice that are insusceptible to MAIDS due to lack of CD40. In contrast, the same infected spleens of CD40-deficient mice contained substantially higher (up to 10-fold) levels of BM5eco gag RNA compared with susceptible controls. Similar to that seen in spleen, infected blood of CD40-deficient mice contained similar levels of BM5def gag as susceptible strains, but increased levels (up to threefold) of BM5eco gag RNA. The assays described below can be used to characterize better the contributions of different functional viral components of the LP-BM5 mixture to the development of MAIDS.

The Role of CD4 T Cells in the Pathogenesis of Murine AIDS

ABSTRACT LP-BM5, a retroviral isolate, induces a disease featuring retrovirus-induced immunodeficiency, designated murine AIDS (MAIDS). Many of the features of the LP-BM5-induced syndrome are shared with human immunodeficiency virus-induced disease. For example, CD4 T cells are critical to the development of MAIDS. In vivo depletion of CD4 T cells before LP-BM5 infection rendered genetically susceptible B6 mice MAIDS resistant. Similarly, MAIDS did not develop in B6.nude mice. However, if reconstituted with CD4 T cells, B6.nude mice develop full-blown MAIDS. Our laboratory has shown that the interaction of B and CD4 T cells that is central to MAIDS pathogenesis requires ligation of CD154 on CD4 T cells with CD40 on B cells. However, it is not clear which additional characteristics of the phenotypically and functionally heterogeneous CD4 T-cell compartment are required. Here, in vivo adoptive transfer experiments using B6.nude recipients are employed to compare the pathogenic abilities of CD4 T-cell subsets defined on the basis of cell surface phenotypic or functional differences. Th1 and Th2 CD4 T cells equally supported MAIDS induction. The rare Thy1.2− CD4 subset that expands upon LP-BM5 infection was not necessary for MAIDS. Interestingly, CD45RBlow CD4 T cells supported significantly less disease than CD45RBhigh CD4 T cells. Because the decreased MAIDS pathogenesis could not be attributed to inhibition by CD45RBlow CD25+ natural T-regulatory cells, an intrinsic property of the CD45RBlow cells appeared responsible. Similarly, there was no evidence that natural T-regulatory cells played a role in LP-BM5-induced pathogenesis in the context of the intact CD4 T-cell population.

Human Immunodeficiency Virus–Specific and CD3-Redirected Cytotoxic T Lymphocyte Activity in the Human Female Reproductive Tract: Lack of Correlation between Mucosa and Peripheral Blood

CD8(+) T cell phenotype and function were assessed in the female reproductive tracts (FRTs) of 3 human immunodeficiency virus (HIV)-positive patients who had undergone hysterectomy. FRT cytotoxic T lymphocyte (CTL) lytic activity from 1 patient (patient 872) was detected by using CD3-dependent redirected-lysis assay and HIV-specific assay, concomitant with the presence of CD8(+) cells. In contrast, samples from the 2 other HIV-positive patients (patients 1356 and 1364), who also were asymptomatic for HIV-associated illnesses, demonstrated no CTL activity in any solid tissue tested by either assay, despite activity by autologous peripheral blood mononuclear cells (PBMC). This absence of CTL activity was correlated with a relative absence of CD8(+) cells in the FRT, whereas CD8(+) cells were present in PBMC. Thus, CTL activity in PBMC may fail to correlate with mucosal activity. The finding of CTL activity in the FRT of patient 872 represents the first description of CTL in upper and lower FRT tissues of an HIV-positive woman.