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The role of CD4+ T cells in the generation of therapeutic primary and memory immune responses in cancer diverse immunotherapy settings remains ambiguous. We herein investigated this issue using two vaccine formulations containing a novel costimulatory molecule, SA-4-1BBL, as adjuvant and HPV E7 or survivin (SVN) as tumor associated antigens (TAAs) in two mouse transplantable tumor models; the TC-1 cervical cancer expressing xenogeneic HPV E7 and 3LL lung carcinoma overexpressing autologous SVN. Single vaccination with optimized SA-4-1BBL/TAA formulations resulted in the eradication of 6-day established TC-1 and 3LL tumors in >70% of mice in both models. The in vivo depletion of CD4+ T cells one day before tumor challenge resulted in compromised vaccine efficacy in both TC-1 (25%) and 3LL (12.5%) tumor models. In marked contrast, depletion of CD4+ T cells 5 days post-tumor challenge and one day prior to vaccination did not significantly alter the therapeutic efficacy of these vaccines. However, long-term immunological memory was compromised in the 3LL, but not in TC-1 model as a significant number (85.7%) of tumor free-mice succumbed to tumor growth when rechallenged with 3LL cells 60 days after the initial tumor inoculation. Collectively, these results demonstrate the indispensable role CD4+ T cells play in the generation of therapeutic primary immune responses elicited by SA-4-1BBL/TAA-based vaccines irrespective of the nature of TAAs and establish the importance of CD4+ T cells for long-term immune memory against 3LL tumor expressing self-antigen SVN, but not TC-1 expressing xenogeneic viral antigen E7. [ABSTRACT FROM AUTHOR]

Subunit vaccines containing universal tumor associated antigens (TAAs) present an attractive treatment modality for cancer primarily due to their safety and potential to generate long-term immunological responses that can safeguard against recurrences. However, TAA-based subunit vaccines require potent adjuvants for therapeutic efficacy. Using a novel form of the 4-1BBL costimulatory molecule, SA-4-1BBL, as the adjuvant of choice, we previously demonstrated that a single vaccination with survivin (SVN) as a bona fide self TAA was effective in eradicating weakly immunogenic 3LL tumors in >70% of C57BL/6 mice. The present study was designed to i) assess the therapeutic efficacy of a prime-boost vaccination and ii) investigate the mechanistic basis of vaccine efficacy. Our data shows that a prime-boost vaccination strategy was effective in eradicating 3LL lung carcinoma in 100% of mice. The vaccine efficacy was correlated with increased percentages of CD8+ T cells expressing IFN-γ as well as potent killing responses of both CD8+ T and NK cells in the absence of detectable antibodies to ssDNA as a sign of autoimmunity. Antibody depletion of CD8+ T cells one day before vaccination completely abrogated therapeutic efficacy, whereas depletion of CD4+ T cells had no effect. Importantly, NK cell depletion had a moderate (~50% reduction), but significant (<0.05) effect on vaccine efficacy. Taken together, these results shed light on the mechanistic basis of the SA-4-1BBL/SVN subunit vaccine formulation in a lung carcinoma model and demonstrate the robust therapeutic efficacy of the prime-boost immunization strategy with important clinical implications. [ABSTRACT FROM AUTHOR]

Tumors convert conventional CD4+ T cells into induced CD4+CD25+FoxP3+ T regulatory (iTreg) cells that serve as an effective means of immune evasion. Therefore, the blockade of conventional CD4+ T cell conversion into iTreg cells represents an attractive target for improving the efficacy of various immunotherapeutic approaches. Using a novel form of 4-1BBL molecule, SA-4-1BBL, we previously demonstrated that costimulation via 4-1BB receptor renders both CD4+ and CD8+ T effector (Teff) cells refractory to inhibition by Treg cells and increased intratumoral Teff/Treg cell ratio that correlated with therapeutic efficacy in various preclinical tumor models. Building on these studies, we herein show for the first time, to our knowledge, that signaling through 4-1BB inhibits antigen- and TGF-β-driven conversion of naïve CD4+FoxP3- T cells into iTreg cells via stimulation of IFN-c production by CD4+FoxP3- T cells. Importantly, treatment with SA-4-1BBL blocked the conversion of CD4+FoxP32 T cells into Treg cells by EG.7 tumors. Taken together with our previous studies, these results show that 4-1BB signaling negatively modulate Treg cells by two distinct mechanisms: I) inhibiting the conversion of CD4+FoxP3- T cells into iTreg cells and ii) endowing Teff cells refractory to inhibition by Treg cells. Given the dominant role of Treg cells in tumor immune evasion mechanisms, 4-1BB signaling represents an attractive target for favorably tipping the Teff:Treg balance toward Teff cells with important implications for cancer immunotherapy. [ABSTRACT FROM AUTHOR]

Tumors convert conventional CD4(+) T cells into induced CD4(+)CD25(+)FoxP3(+) T regulatory (iTreg) cells that serve as an effective means of immune evasion. Therefore, the blockade of conventional CD4(+) T cell conversion into iTreg cells represents an attractive target for improving the efficacy of various immunotherapeutic approaches. Using a novel form of 4-1BBL molecule, SA-4-1BBL, we previously demonstrated that costimulation via 4-1BB receptor renders both CD4(+)and CD8(+) T effector (Teff) cells refractory to inhibition by Treg cells and increased intratumoral Teff/Treg cell ratio that correlated with therapeutic efficacy in various preclinical tumor models. Building on these studies, we herein show for the first time, to our knowledge, that signaling through 4-1BB inhibits antigen- and TGF-β-driven conversion of naïve CD4(+)FoxP3(-) T cells into iTreg cells via stimulation of IFN-γ production by CD4(+)FoxP3(-) T cells. Importantly, treatment with SA-4-1BBL blocked the conversion of CD4(+)FoxP3(-) T cells into Treg cells by EG.7 tumors. Taken together with our previous studies, these results show that 4-1BB signaling negatively modulate Treg cells by two distinct mechanisms: i) inhibiting the conversion of CD4(+)FoxP3(-) T cells into iTreg cells and ii) endowing Teff cells refractory to inhibition by Treg cells. Given the dominant role of Treg cells in tumor immune evasion mechanisms, 4-1BB signaling represents an attractive target for favorably tipping the Teff:Treg balance toward Teff cells with important implications for cancer immunotherapy.