4-1BBL Enhances Anti-tumor Responses in the Presence or Absence of CD28 but CD28 Is Required for Protective Immunity against Parental Tumors

A20 is an aggressive BALB/c B cell lymphoma that, despite its expression of B7-2, rapidly forms tumors in syngeneic mice. We have generated A20 transfectants expressing elevated levels of B7-2 (A20/B7-2high) or 4-1BBL (A20/4-1BBLlow,mod,high) and found that mice which were able to reject the A20/B7-2 or A20/4-1BBL transfectants were also resistant to subsequent systemic challenge with the parental cell line. To assess whether the effectiveness of 4-1BBL in enhancing anti-tumor immunogenicity was dependent on additional signals from B7-CD28 interaction, we injected the A20 variants into BALB/c CD28−/− mice. We found that CD28−/− mice were able to reject the A20/4-1BBL variants while A20/B7-2 cells formed tumors. However, when the A20/4-1BBL resistant CD28−/− mice were systemically challenged with the A20 parental line, tumors formed rapidly. Upon restimulation in vitro, splenocytes from A20/4-1BBL immunized CD28+/+ mice were able to kill parental tumors whereas splenocytes from CD28−/− mice showed a reduction in CTL activity against A20 or A20/4-1BBL targets. Examination of cytokine production by the immunized animals indicated that the CD28−/− splenocytes secreted substantially less IL-2 as well as reduced levels of IFN-γ compared with their CD28+/+ counterparts. Thus, 4-1BBL expressing tumors are capable of priming CTL responses against 4-1BBL transfected as well as parental tumors in the absence of CD28. However, in the absence of CD28 signaling, the production of cytokines and particularly IL-2 was lower, resulting in a weaker CTL recall response and reduced ability to survive challenge with parental tumor.