REPEATED TREATMENT WITH ANTIBODY-TARGETED SUPERANTIGENS STRONGLY INHIBITS TUMOR-GROWTH

Superantigens (SAg) are microbial proteins with the capacity to activa te a large proportion of T cells. We have developed a novel approach f or cancer immunotherapy by genetically fusing the SAg staphylococcal e nterotoxin A (SEA) to a Fab-fragment of a tumor-specific antibody. Rep eated exposure to SEA induces a state of unresponsiveness including ce ll deletion and functional hyporesponsiveness, i.e., anergy. In this s tudy we have developed improved therapeutic schedules to allow repeate d injections of Fab-SEA, limit development of immunological unresponsi veness and promote maximal anti-tumor response. Four daily injections of Fab-SEA to mice carrying B16-C215 lung metastases resulted in 90-95 % reduction in the number of metastases. However, the animals did reta in a minimal residual tumor disease. The immune system was in a hypore sponsive state after 4 daily Fab-SEA injections, and further injection s did not improve therapy. Two repeated cycles, each comprising 4 dail y injections of Fab-SEA, significantly prolonged the survival and resu lted in complete cure of a fraction of the animals. A rest period of I n days between the cycles was required to mount an efficient secondary anti-tumor response. This secondary immune response was characterized by partial recovery of cytokine production ie., interleukin-2, interf eron-gamma and tumor necrosis factor-alpha. Strong CTL activity was de tected in animals that had rested for 8 weeks between the 2 cycles. In terestingly, irrespective of the resting period, the CD4(+) SEA-reacti ve T cells expanded in response to all 4 additional Fab-SEA injections both locally and in spleen. In contrast, only marginal expansion of C D8(+) T cells was seen if restimulation was given within I month. Our data show that potent anti-tumor effector functions can be induced aft er repeated stimulation cycles with a SAg-monoclonal antibody fusion p rotein resulting in a CD4(+) T cell-dependent cytokine release, prolon ged survival and induction of complete cures. (C) 1998 Wiley-Liss, Inc .