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 non-specific activation of lymphocytes. We hypothesized that natural costimulatory
ligands may serve as more potent and safer alternative to agonistic Abs for
immunotherapy. In this communication, 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 naïve 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 C1q(-/-) or C3(-/-) knockout mice, suggesting lack of involvement of stimulatory FcgammaRs or
complement system in the observed toxicity. Naïve and memory T cells served as direct targets of anti-4-1BB Ab-mediated toxicity. Potent immunostimulatory activity combined with lack of toxicity rationalizes further development of soluble SA-4-1BBL as an immunomodulatory component of therapeutic
vaccines against
cancer and
chronic infections.