The role of natural killer cell mediated caspases activation in a graft-versus-host disease model of semiallogeneic small bowel transplantation

In the clinical setting of solid organ transplantation the event of graft-v ersus-host disease (GvHD) is rare and not easily predictable. Even intestin al and multivisceral transplants harbour a huge amount of immunocompetent c ells and they do not exert a significantly higher risk to trigger serious G vH reactions. A series of our own experimental studies has been conducted t o delineate the role of the host's innate immune system in the context of G VHD following parental to F1 hybrid semiallogeneic small bowel transplantat ion (SBTx). These results clearly demonstrated the immunological significan ce of the recipient's status of natural killer (NK) cell activity to counte ract donor-derived lymphocytes and related cytotoxicity. NK cells and macro phages are both endowed with Ca2+-dependent receptors of the C-type lectin family which interact with a diversity of high-affinity oligosaccharide lig ands expressed on potential target cells. One of these proteins of the C-ty pe lectin family, termed NKR-P1, has been cloned and sequenced. Activation of NKR-P1 stimulates activation-induced cell death (AICD) of bound target c ells. As intracellular mediators of apoptotic cell death a new family of cy steine proteases, the caspases, have been defined. These proteases appear t o be involved in the initiation of apoptosis in response to a number of sti muli. This study was conducted to investigate the impact on the activity level of host MK cells and on target cell lysis of donor-derived lymphocytes after heterotopic semiallogeneic (parental [DA;RT1.(aaav1)] to F1 [DA x LEW;RT1.( 1)]) small bowel transplantation using a rat model. The host's NK activity was either specifically activated (by use of polyinosinic:polycytodilic aci d [poly-I:C]) or suppressed (by depletion of host NK cells after intraperit oneal administration of the NKR-P1 monoclonal antibody 3.2.3). The impact o f NK-activity on the incidence of GVHD and the recipients' survival was cor related with the frequency of apoptotic cell death and related expression o f caspases 1 (ICE) and 3 (CPP-32) from donor and recipient small bowel tiss ues. Our results confirm that depletion of NK cells in F1 host rats prior to par ental small bowel transplantation significantly decreased the mean survival to 11.4 days versus 16.2 days of nondepleted F1 rats (p < 0.01). Conversel y, activation of host NK activity with poly-I:C abrogated GvHD in all 12 re cipient rats and led to long-term survival in seven of 12 animals. Long-ter m survival was associated with a substantially higher frequency of apoptoti c cell death in donor and recipient small bowel and mesenteric lymph nodes. On day 10 after transplantation, Northern blot analysis of these tissues r evealed profound upregulation of mRNA-specific gene expression for caspase 1 and 3 as potential mediators of programmed cell death of activated lympho cytes. Our findings emphasize the importance of NK cell associated innate immunity in the context of GvHD after semiallogeneic small bowel transplantation. K illing of alloreactive donor-derived lymphocytes was mediated by the NKR-P1 protein on NK cells and could be suppressed after pretreatment of Fl hosts with anti-NKR-P1 mAb 3.2.3. Moreover, NX cell-mediated apoptosis induced u pregulation of caspases 1 and 3, thus elucidating the involvement of this p rotein in the context of caspase-mediated target cell killing.