Engineering MHC class I-negative pig cells: effects on human cellular immune responses
Direct recognition of porcine MHC proteins by human T cells is well documented. Eliminating donor (porcine) MHC proteins may therefore be beneficial in pig to human xenotransplants. To this end, we have attempted to exploit viral stealth mechanisms to eliminate pig MHC class I cell-surface expression. PK(15) (pig kidney) cells stably transfected with the Herpes Simplex Virus (HSV) ICP47 gene (PK(15)-ICP47 cells) exhibited a dramatic reduction of MHC class I cell-surface expression when compared to untransfected PK(15) cells. To test the effect of down-regulation of porcine MHC class I on human cellular immune responses, a human CD8+ T cell line (MDC anti-PK) with reactivity towards PK(15) cells was derived by repeated stimulation of human T cells with PK(15) cells stably transfected with the costimulatory molecule B7.1 (PK(15)-B7.1 cells). MDC anti-PK T cells efficiently lysed PK(15) cells but not PK(15)-ICP47 (class I negative) cells. Consistent with effector function, MDC anti-PK cells showed a robust proliferative response to PK(15)-B7.1 cells but did not proliferate at all to PK(15)-B7.1 cells which also expressed HSV ICP47. The class I-negative PK(15) cell line, PK(15)-ICP47, afforded a unique opportunity to address the role of porcine MHC class I proteins in the susceptibility to human NK cell-mediated lysis. While PK(15) cells were moderately susceptible to human NK cell-mediated lysis, class I deficient PK(15) cells (PK(15)-ICP47) were highly susceptible. These results imply that porcine MHC class I proteins deliver at least some inhibitory signal(s) to human NK cells. Taken together the findings suggest that while eliminating porcine MHC class I cell-surface proteins may reduce human T cell recognition of xenografts there could be negative consequences in terms of NK cell mediated rejection.