A radiosensitive APC activity dissociates IL-2 secretion and activation-induced cell death by autoreactive T cell hybridomas

Immune-priming of the tumor microenvironment by radiotherapy: rationale for combination with immunotherapy to improve anticancer efficacy

A clear contribution of the immune system to eradication of tumors has been supported by recent developments in the field of immunotherapy. Durable clinical responses obtained after treatment with immunomodulatory agents such as ipilimumab (Yervoy) and anti-PD-1 antibody (BMS-936558), have established that harnessing the immune response against chemoresistant tumors can result in their complete eradication. However, only a subset of patients benefit from these therapeutic approaches. Accumulating evidence suggests that tumors with a preexisting active immune microenvironment might have a better response to immunotherapy. In a number of preclinical and clinical studies, many cytotoxic agents elicit changes within tumors and their microenvironment that may make these malignant cells more sensitive to an efficient immune cell attack. Therefore, it is plausible that combining immunotherapy with standard anticancer therapies such as chemotherapy or radiotherapy will provide synergistic antitumor effects. Despite a large collection of preclinical data, the immune mechanisms that might contribute to the efficacy of conventional cytotoxic therapies and their combinations with immunotherapeutic approaches have not yet been extensively studied in the clinical setting and warrant further investigation. This review will focus on current knowledge of the immunomodulatory effects of one such cytotoxic treatment, radiotherapy, and explore different pathways by which its combination with immunomodulatory antibodies might contribute toward more efficacious antitumor immunity.

Antigen presentation in uveitis

Experimental autoimmune uveoretinits (EAU) is not only a valuable model for human inflammatory eye diseases, it is also a useful system for studying many aspects of immunobiology. One such aspect is self/non-self discrimination, the ability of the immune system to tolerate self molecules while responding aggressively to foreign antigens. Our laboratory has used EAU to investigate the mechanisms of T cell tolerance to retinal S-antigen (S-Ag). Several mechanisms have been proposed to maintain T cell tolerance to autoantigens, including clonal deletion and clonal anergy. As immunisation with S-Ag or pathogenic peptides activates uveitogenic T cells, tolerance to this autoantigen cannot be due to clonal deletion. Nevertheless, tolerance acts to keep these existing autoreactive T cells in a naive, or innocuous state. Here we suggest a novel mechanism--low-affinity occupancy of the autoantigen-specific T cell receptor (TCR) by self-antigen--that may act in concert with the well-known mechanisms to maintain tolerance to S-Ag in the LEW rat. This model differs from clonal anergy in that the missing antigen-presenting cell (APC) activity is not a co-stimulatory function but a TCR co-ligand that increases the avidity of the interaction between the TCR and its peptide-major histocompatibility complex (MHC) ligand. In the absence of this co-ligand only partial signals are generated through the TCR, leading to incomplete T cell activation. This model was deduced from experiments with T cell lines and hybridomas specific for S-Ag, which showed that: (1) autoreactive T cells required a novel APC function, (2) this novel function was necessary to provide complete TCR engagement, and (3) activation of autoreactive T cells was restricted to specific APC.

Trypanosoma cruzi-infected macrophages are defective in major histocompatibility complex class II antigen presentation

Trypanosoma cruzi, the intracellular protozoan parasite that causes Chagas' disease, interferes with the host immune response to establish a persistent infection. In this report, we demonstrate that macrophages infected with T. cruzi are unable to effectively present antigens to CD4 T cells. The interference is due to defective antigen-presenting cell (APC) function, as antigen-independent stimulation of the T cell in the presence of infected macrophages is not affected. The defect is distal to antigen processing and is not due to decreased major histocompatibility complex (MHC) class II expression, decreased viability, defective peptide loading in the infected macrophages, nor absence of CD28 co-stimulation. There was a role for gp39: CD40 co-stimulation during antigen presentation to the T cells we studied, but the expression of CD40 on T. cruzi-infected macrophages was not decreased. Antigen-specific adhesion between macrophages and T cells was reduced by infection. Equivalent levels of the adhesion molecules lymphocyte function-associated antigen-1, intercellular adhesion molecule-1, vascular cell adhesion molecule-1 or very late antigen-4 are found on infected and uninfected APC, suggesting that reduced expression of these adhesion molecules was not responsible for the defect in antigen-specific adhesion. The defective T cell:macrophage adhesion may be due to the reduced expression of other adhesion molecules or other changes in the cell induced by infection. Interfering with MHC class II antigen presentation in infected macrophages may help T. cruzi to blunt the immune response by the host.

Differential APC requirements of self- and nonself-reactive T cells and T cell hybridomas specific for retinal S-antigen

Autoreactive T cell hybridomas specific for pathogenic peptides of retinal S-Ag require a novel radiosensitive APC activity for IL-2 secretion that is distinct from Ag presentation by MHC class II. Antigen-dependent IL-2 secretion by self-reactive hybridomas was much more efficient with splenic APC than with thymic APC, although both provided similar levels of hybridoma TCR occupancy as measured by activation-induced cell death. Furthermore, thymic APC did stimulate IL-2 secretion by a non-self reactive hybridoma. To test the hypothesis that this activity was provided by a distinct cell population, fractionated splenocytes were tested for their ability to present Ag to these hybridomas. The most potent Ag presentation for IL-2 secretion was found to segregate with low-density, B cell-enriched fractions while adherent cells, or purified T cells were unable to support IL-2 production. Together with previous results, the data show that antigen presentation leading to IL-2 secretion by these autoreactive T cell hybridomas requires activated B cells, whereas TCR occupancy can be provided by several APC subsets.