| Costimulatory molecules of the CD28 and TNFR superfamilies play critical roles in modulating innate, adaptive, and regulatory immune reponses. As such, agonistic ligands to these receptors have the potential to serve as effective immunomodulatory components of vaccines. Indeed, agonistic Abs to select costimulatory members of CD28 and TNFR family have shown efficacy in various preclinical cancer immunotherapeutic settings. However, the use of agonistic Abs is often associated with severe toxicity due to nonspecific activation of lymphocytes. We hypothesized that natural costimulatory ligands may serve as more potent and safer alternative to agonistic Abs for immunotherapy. We focused on 4-1BBL as the molecule of choice because of the pleiotropic effects of 4-1BB signaling in the immune system and the demonstrated therapeutic efficacy of 4-1BB agonistic Abs in preclinical cancer and infection models. We report that a novel form of soluble ligand, SA-4-1BBL, delivered more potent and qualitatively different signals to T cells than an agonistic Ab. Importantly, while treatment of naive mice with the agonistic Ab resulted in severe toxicity, as assessed by enlarged spleen and peripheral LNs, non-specific T cell proliferation, hepatitis, and systemic inflammatory cytokine production, treatment with SA-4-1BBL lacked these immune anomalies. Agonistic Ab treatment produced full toxicity in FcgammaR-/- or complement Clq-/- or C3-/- knockout mice, suggesting lack of involvement of stimulatory FcgammaRs or complement system in the observed toxicity. Naive and memory T cells served as direct targets of anti-4-1BB Ab mediated toxicity. Additionally, we demonstrated that the pleitropic effects of SA-4-1BBL on innate, adaptive, and regulatory immunity translate into effective cancer immunotherapy when SA-4-1BBL was also used as a vehicle to deliver antigens in vivo to DCs constitutively expressing the 4-1BB receptor. SA-4-1BBL was internalized by a subpopulation of DCs upon receptor binding, and immunization with biotinylated antigens conjugated to SA-4-1BBL resulted in increased antigen uptake and cross-presentation by DCs, leading to the generation of effective immune responses. Potent immunostimulatory activity and lack of toxicity combined with the ability to serve as a vehicle to increase delivery of antigens to DCs in vivo rationalizes further development of soluble SA-4-1BBL as an immunomodulatory component of therapeutic vaccines against cancer and chronic infections. |
