4-1BB costimulation is required for protective anti-viral immunity after peptide vaccination

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 (A20/4-1BBL(low,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-gamma 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.

Role of TNF receptor-associated factor 2 and p38 mitogen-activated protein kinase activation during 4-1BB-dependent immune response

4-1BB is a costimulatory member of the TNFR family, expressed on activated CD4(+) and CD8(+) T cells. Previous results showed that 4-1BB-mediated T cell costimulation is CD28-independent and involves recruitment of TNFR-associated factor 2 (TRAF2) and activation of the stress-activated protein kinase cascade. Here we describe a role for the p38 mitogen-activated protein kinase (MAPK) pathway in 4-1BB signaling. Aggregation of 4-1BB alone induces p38 activation in a T cell hybridoma, whereas, in normal T cells, p38 MAPK is activated synergistically by immobilized anti-CD3 plus immobilized 4-1BB ligand. 4-1BB-induced p38 MAPK activation is inhibited by the p38-specific inhibitor SB203580 in both a T cell hybridoma and in murine T cells. T cells from TRAF2 dominant-negative mice are impaired in 4-1BB-mediated p38 MAPK activation. A link between TRAF2 and the p38 cascade is provided by the MAPK kinase kinase, apoptosis-signal-regulating kinase 1. A T cell hybrid transfected with a kinase-dead apoptosis-signal-regulating kinase 1 fails to activate p38 MAPK in response to 4-1BB signaling. To assess the role of p38 activation in an immune response, T cells were stimulated in an MLR in the presence of SB203580. In a primary MLR, SB203580 blocked IL-2, IFN-gamma, and IL-4 secretion whether the costimulatory signal was delivered via 4-1BB or CD28. In contrast, following differentiation into Th1 or Th2 cells, p38 inhibition blocked IL-2 and IFN-gamma without affecting IL-4 secretion. Nevertheless, IL-4 secretion by Th2 cells remained costimulation-dependent. Thus, critical T cell signaling events diverge following Th1 vs Th2 differentiation.

Tumour rejection by gene transfer of 4-1BB ligand into a CD80(+) murine squamous cell carcinoma and the requirements of co-stimulatory molecules on tumour and host cells

NRS1 is a murine squamous cell carcinoma that constitutively expresses the co-stimulatory molecule CD80 at a high level yet grows as a tumour in syngeneic C3H mice. We examined the effect of gene transfer of the 4-1BB ligand (4-1BBL) into NRS1 cells. Introduction of the 4-1BBL gene efficiently elicited anti-tumour immune responses in syngeneic mice which acquired specific immunity against wild-type tumour. T-cell depletion studies showed that CD8(+), but not CD4(+) T cells were essential for tumour eradication. Our results suggest that the transduced 4-1BBL is more effective than the spontaneously expressed CD80 for generation of primary anti-tumour CD8(+) T-cell responses. In addition to CD80 and CD86, the host-derived 4-1BBL is also involved in the secondary anti-tumour responses. This study indicates the complicated contribution of 4-1BBL, CD80 and CD86 on tumour and host cells in anti-tumour immune responses and a possible therapeutic application of 4-1BBL for human tumour vaccination and gene therapy.

Costimulation in antiviral immunity: differential requirements for CD4(+) and CD8(+) T cell responses

A key step toward improving vaccines is understanding the molecular interactions responsible for inducing antiviral T cell responses. An emerging theme from recent studies is that CD4(+) and CD8(+) T cell responses require distinct costimulatory pathways for activation. In addition, these costimulatory interactions can play a crucial role during the death phase of the T cell response and determine the number of effector T cells that survive to become memory T cells.