Demonstration of cell-mediated cytotoxicity to allogeneic and xenogeneic tissue in the American cockroach, Periplaneta americana, using a combination in vivo/in vitro assay.

Previous reports have claimed that insects lack the ability to reject integumentary allografts. However, when we followed the fate of the epidermal layer underneath the inert cuticle, we observed that the American cockroach not only rejected xenografts, but also demonstrated impressive reactivity against allografts. We have now developed a combination in vivo/in vitro assay that can quantitate the extent of allograft reactivity in the roach. Pieces of filter paper (3 X 4-mm) were implanted under the second and fourth tergites of donor animals. After 7 days, implants seeded with host hemocytes were removed, washed, and incubated in culture medium containing 3H-thymidine for 24 hr. Labeled grafts were reimplanted into paired animals to detect cytotoxicity as follows: grafts removed from under the second tergite were placed back into their original positions to serve as autograft controls; grafts removed from under the fourth tergite were reciprocally transferred between paired animals; and grafts were recovered after various time intervals and processed for scintillation counting. There was no significant difference in counts between allografts and autografts at day 1. Autografts sampled on days 3, 5, 7, and 10 had significantly higher counts than allografts, with peak reactivity occurring between days 3 and 5. This indicates that allogeneic cells were selectively destroyed, and confirms our recent data from conventional grafting studies that insects have the ability to react to allografts.