Direct T Cell Activation via CD40 Ligand Generates High Avidity CD8+ T Cells Capable of Breaking Immunological Tolerance for the Control of Tumors

Abstract

CD40 and CD40 ligand (CD40L) are costimulatory molecules that play a pivotal role in the proinflammatory immune response. Primarily expressed by activated CD4+ T cells, CD40L binds to CD40 on antigen presenting cells (APCs), thereby inducing APC activation. APCs, in turn, prime cytotoxic T lymphocytes (CTLs). Here, two tumor-associated antigen (TAA) animal models, p53-based and GP100-based, were utilized to examine the ability of CD40-CD40L to improve antigen-specific CTL-mediated antitumor immune responses. Although p53 and GP100 are self-antigens that generate low affinity antigen-specific CD8+ T cells, studies have shown that their functional avidity can be improved with CD40L-expressing APCs. Therefore, in the current study, we immunized mice with a DNA construct encoding a TAA in conjunction with another construct encoding CD40L via intramuscular injection followed by electroporation. We observed a significant increase in the antigen-specific CTL-mediated immune responses as well as the potent antitumor effects in both models. Antibody depletion experiments demonstrated that CD8+ T cells play a crucial role in eliciting antitumor effects in vaccinated mice. Furthermore, we showed that in vitro stimulation with irradiated tumor cells expressing both TAA and CD40L improved the functional avidity of antigen-specific CD8+ T cells. Thus, our data show that vaccination with TAA/CD40L DNA can induce potent antitumor effects against TAA-expressing tumors through the generation of better functioning antigen-specific CD8+ T cells. Our study serves as an important foundation for future clinical translation.

Distinct Graft-Specific TCR Avidity Profiles during Acute Rejection and Tolerance

“…For these other specificities, resistance to avidity maturation may be epigenetically imprinted, similar to T cell-intrinsic hyporesponsiveness, which makes cells resistant to secondary encounter of antigen (Philip et al, 2017; Schietinger et al, 2012), or may be constrained by regulatory cells distinct from classical Tregs. Studies of T cells in tumor microenvironments that are deemed dysfunctional and of self-specific T cells (Black et al, 2014; Kuball et al, 2009; Malhotra et al, 2016; Moon et al, 2011; Soong et al, 2014; Souders et al, 2007; Wong et al, 2008; Yu et al, 2015) have also revealed that many of these cells are low avidity at the population level, supporting the idea that tolerance, whether spontaneously acquired in the tumor microenvironment or upon self-antigen encounter or, as we show in this study, therapeutically induced by costimulation blockade therapy, may be simultaneously enforced through multiple mechanisms, one of which being the preservation of low-avidity repertoire for the relevant antigen.…”

“…There are strong correlations between accumulation of highavidity T cells and clearance of infections (Busch and Pamer, 1999), elimination of tumors (Black et al, 2014; Kuball et al, 2009; Soong et al, 2014), better memory responses (Turner et al, 2008; Zehn et al, 2009), and autoimmunity (Maeda et al, 2014). Low-avidity T cells, however, which are correlated with less effector functionality (Falta et al, 2005; Kuball et al, 2009; Tsang et al, 2011), would be beneficial for transplantation.…”

DNA vaccine for cancer immunotherapy

“…Significant increase in antigen specific CTL cellular immune responses was observed in both p53 and gp100 tumor models. 90 Furthermore, the immune responses were shown to be CD8+ T cell mediated and result in potent antitumor response. The results show that immunostimulatory molecules can be incorporated into DNA vaccines to enhance the vaccines’ potency.…”

Potent Activation of Human T Cells by mRNA Encoding Constitutively Active CD40

“…A growing number of studies, most of which have been performed in mice, suggest that CD40 is functionally expressed also by T cells. The direct T cell stimulatory capacity of CD40 was manifested in a wide range of effects, including differentiation, memory formation, improvement of functional avidity, upregulating antiapoptotic signals and decreasing proapoptotic ones, rescue from exhaustion, and acquisition of resistance to regulatory T cell-mediated suppression (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). Yet, other studies failed to confirm these observations (24)(25)(26)(27), and the immunological role played by T cell-expressed CD40 under physiological conditions is still elusive.…”