Role of toll-like receptors and their adaptors in adjuvant immunotherapy for cancer

Abstract

The potentiation of immune responses to tumor-associated antigen (Ag) is a pivotal issue in immunotherapy for cancer and thus requires the use of adjuvants, which are involved in efficient antibody (Ab) production and killer cell induction. The efficacy for tumor regression of a number of adjuvants that have been applied to immunotherapy in humans and tumor-bearing animal models has been tested without understanding of the function of adjuvants. Recent findings on the function of Toll-like receptors (TLRs) and their adaptors facilitated the elucidation of the molecular basis of adjuvant activity. TLR signaling was found to induce interferons (IFNs), chemokines and proinflammatory cytokines and mature dendritic cells (DCs) for enhanced efficiency in antigen presentation. The mediators then play a crucial role in the organization of acquired immunity and, together with matured DCs, activate cytotoxic T cells (CTL) and NK cells. These TLR outputs vary among adjuvants, which may depend on adjuvant-specific selection of appropriate sets of TLRs and their adaptors. Here we review how a variety of host immune responses are induced by an individual adjuvant to confer an adjuvant-specific anti-tumor immunity. We elaborate specifically on two adjuvants, BCG-cell wall skeleton and double-stranded RNA (dsRNA). The former activates TLR2/4 on DCs and induces tumor-specific CTL allowing general application to patients with surgically dissected cancer and improving prognosis, while the latter activates TLR3 on DCs to release type 1 IFN that induces tumor cell apoptosis and NK-mediated tumor cytotoxicity.