Dynamic control of self-specific CD8+ T cell responses via a combination of signals mediated by dendritic cells

IFNγ-Dependent Interactions between ICAM-1 and LFA-1 Counteract Prostaglandin E2-Mediated Inhibition of Antitumor CTL Responses

Tumor-expressed ICAM-1 interaction with LFA-1 on naïve tumor-specific CD8(+) T cells not only stabilizes adhesion, but, in the absence of classical B7-mediated costimulation, is also able to provide potent alternative costimulatory signaling resulting in the production of antitumor cytotoxic T lymphocyte (CTL) responses. This study shows that overproduction of prostaglandin (PG) E2 by metastatic murine renal carcinoma (Renca) cells inhibited direct priming of tumor-specific CTL responses in vivo by preventing the IFNγ-dependent upregulation of ICAM-1 that is vital during the initial priming of naïve CD8(+) T cells. The addition of exogenous IFNγ during naïve CD8(+) T-cell priming abrogated PGE2-mediated suppression, and overexpression of ICAM-1 by tumor cells restored IFNγ production and proliferation among PGE2-treated tumor-specific CD8(+) T cells; preventing tumor growth in vivo These findings suggest that novel anticancer immunotherapies, which increase expression of ICAM-1 on tumor cells, could help alleviate PGE2-mediated immunosuppression of antitumor CTL responses. Cancer Immunol Res; 4(5); 400-11. ©2016 AACR.

Dendritic cells: cellular mediators for immunological tolerance

In general, immunological tolerance is acquired upon treatment with non-specific immunosuppressive drugs. This indiscriminate immunosuppression of the patient often causes serious side-effects, such as opportunistic infectious diseases. Therefore, the need for antigen-specific modulation of pathogenic immune responses is of crucial importance in the treatment of inflammatory diseases. In this perspective, dendritic cells (DCs) can have an important immune-regulatory function, besides their notorious antigen-presenting capacity. DCs appear to be essential for both central and peripheral tolerance. In the thymus, DCs are involved in clonal deletion of autoreactive immature T cells by presenting self-antigens. Additionally, tolerance is achieved by their interactions with T cells in the periphery and subsequent induction of T cell anergy, T cell deletion, and induction of regulatory T cells (Treg). Various studies have described, modulation of DC characteristics with the purpose to induce antigen-specific tolerance in autoimmune diseases, graft-versus-host-disease (GVHD), and transplantations. Promising results in animal models have prompted researchers to initiate first-in-men clinical trials. The purpose of current review is to provide an overview of the role of DCs in the immunopathogenesis of autoimmunity, as well as recent concepts of dendritic cell-based therapeutic opportunities in autoimmune diseases.

Priming CD8+ T cells with dendritic cells matured using TLR4 and TLR7/8 ligands together enhances generation of CD8+ T cells retaining CD28

TLRs expressed on dendritic cells (DCs) differentially activate DCs when activated alone or in combination, inducing distinct cytokines and costimulatory molecules that influence T-cell responses. Defining the requirements of DCs to program T cells during priming to become memory rather than effector cells could enhance vaccine development. We used an in vitro system to assess the influence of DC maturation signals on priming naive human CD8+ T cells. Maturation of DCs with lipopolysaccharide (LPS; TLR4) concurrently with R848 (TLR7/8) induced a heterogeneous population of DCs that produced high levels of IL12 p70. Compared with DCs matured with LPS or R848 alone, the DC population matured with both adjuvants primed CD8+ T-cell responses containing an increased proportion of antigen-specific T cells retaining CD28 expression. Priming with a homogenous subpopulation of LPS/R848-matured DCs that were CD83(Hi)/CD80+/CD86+ reduced this CD28+ subpopulation and induced T cells with an effector cytokine signature, whereas priming with the less mature subpopulations of DCs resulted in minimal T-cell expansion. These results suggest that TLR4 and TLR7/8 signals together induce DCs with fully mature and less mature phenotypes that are both required to more efficiently prime CD8+ T cells with qualities associated with memory T cells.

TNFR1 signalling is a critical checkpoint for developing macrophages that control of T-cell proliferation

Macrophages (Mϕ) are professional antigen-presenting cells, but when they accumulate at sites of inflammation, they can inhibit T-cell proliferation. In experimental autoimmune uveoretinitis, this limits the expansion of T cells within the target organ. To define requirements for the elaboration of this outcome, we have generated populations of Mϕ in vitro that could also regulate T-cell responses; stimulating CD4(+) T-cell activation and cytokine production, but simultaneously suppressing T-cell proliferation. When T cells are removed from the influence of such cells, normal T-cell responses are restored. We show that tumour necrosis factor 1 (TNFR1) signalling is a critical checkpoint in the development of such Mϕ, as TNFR1(-/-) Mϕ are unable to suppress T-cell proliferation. This deficit in antigen-presenting cells results in a lack of production of prostaglandin E(2) (PGE(2)) and nitric oxide, which are critical effector mechanisms that inhibit T-cell division. However, TNFR1 signalling is not required for the inhibitory function of Mϕ because we could circumvent the requirement for this receptor, by maturing Mϕ in the presence of exogenous interferon-γ and PGE(2). This produced TNFR1(-/-) Mϕ that inhibited T-cell proliferation and indicates that TNFR1 delivers a signal that is necessary for the development but not the execution of this function.

Prevention of both direct and cross-priming of antitumor CD8+ T-cell responses following overproduction of prostaglandin E2 by tumor cells in vivo

Defects in antitumor immune responses have been associated with increased release of prostaglandin E(2) (PGE(2)) as a result of overexpression of cyclooxygenase (COX)-2 by tumors. In this report, we examine the effects of PGE(2) on antitumor CD8(+) T-cell responses generated both by cross-presenting dendritic cells and by direct priming by tumor cells. Our data show that PGE(2) inhibits dendritic cell maturation, resulting in the abortive activation of naive CD8(+) T cells, and is dependent on interleukin-10 production by dendritic cells. Interaction of tumor cells with naïve CD8(+) T cells in the presence of PGE(2) in vitro results in the induction of CD8(+) CD28(-) T cells, which fail to proliferate or exhibit effector function. In vivo, overexpression of COX-2 by tumor cells results in a decrease in number of tumor-infiltrating dendritic cells and confers the ability of tumor cells to metastasize to the tumor draining lymph nodes.

Loss of CTL function among high-avidity tumor-specific CD8+ T cells following tumor infiltration

A major problem in generating effective antitumor CTL responses is that most tumors express self-antigens to which the immune system is rendered unresponsive due to mechanisms of self-tolerance induction. CTL precursors (CTLp) expressing high-affinity T-cell receptors (TCR) are often functionally deleted from the repertoire, leaving a residual repertoire of CTLp having only low-affinity TCR. Furthermore, even when unique antigens are expressed, their presentation by dendritic cells (DC) may predispose to peripheral tolerance induction rather than the establishment of CTL responses that kill tumor cells. In this study, we examined both high-avidity (CL4) and low-avidity (CL1) CD8(+) T-cell responses to a murine renal carcinoma expressing, as a neoantigen, high and low levels of the hemagglutinin (HA) protein from influenza virus A/PR/8 H1N1 (PR8; RencaHA(high) and RencaHA(low)). Our data show that, following encounter with K(d)HA epitopes cross-presented by bone marrow-derived DC, low-avidity CL1 cells become tolerized within tumor-draining lymph nodes (TDLN), and in mice bearing either RencaHA(high) or RencaHA(low) tumors, very few form tumor-infiltrating lymphocytes (TIL). In marked contrast, high-avidity CL4 cells differentiate into effector CTL within the TDLN of mice bearing either RencaHA(high) or RencaHA(low) tumors, and although they form TIL in both tumors, they lose CTL effector function. Critically, these results show that anticancer therapies involving either adoptive transfer of high-avidity tumor-specific CTL populations or targeting of preexisting tumor antigen-specific memory CD8(+) T cells could fail due to the fact that CTL effector function is lost following tumor infiltration.