A20 is a
B cell lymphoma that constitutively expresses the
costimulatory molecule B7-2 yet grows readily as a
tumor in syngeneic BALB/c mice. We have compared the tumorigenicity of A20 variants expressing either B7-1 (A20/B7-1) or B7-2 (A20/B7-2) with an A20 variant expressing B7-1 and B7-2 with 4-1BBL (A20/4-1BBL), a costimulatory member of the TNF family. Mice injected with
tumors expressing the vector backbone (A20/CMV) or B7-1 developed
tumors within 25 days of s.c. injection. In contrast, mice injected with A20/4-1BBL were
tumor free for the 150-day follow-up period, while 25% of mice injected with A20/B7-2 developed
tumors. Tumorigenicity experiments using nude mice indicated the requirement for T cells for variant rejection. Almost all mice that resisted the initial
tumor challenge were resistant to further challenge with the parental
tumor. Splenocytes from these mice showed high CTL lytic activity against the parental
tumor, A20, as well as the syngeneic BALB/c
lymphoma K46J, but showed background levels of lytic activity against the congenic SCID
thymoma line ST-D2 or the allogeneic EL4
thymoma. In vitro blocking experiments with anti-B7-1 plus anti-B7-2 and/or soluble
4-1BB receptor showed B7-1, B7-2, and 4-1BBL all contributed to the CTL activity. Thus, the data show that neither B7-1 or B7-2 alone can confer full immunogenicity to the A20
lymphoma but that the addition of 4-1BBL results in a
tumor that is highly immunogenic and can confer long-lasting protection against challenge with parental
tumor in vivo.