Though experience with allogeneic stem cell transplant for osteosarcoma is limited, suggestion of a graft-vs.-tumor effect was observed in a male with metastatic osteosarcoma who received an HLA-matched sibling transplant and achieved long-term survival[20
]. Osteosarcoma may be a good candidate disease for KIR incompatible transplants as evidenced by recent observations of loss or down-regulation of MHC class I antigens in 52% of primary osteosarcoma tumors and in 88% of metastatic osteosarcoma specimens[13
]. Our study shows variable MHC class I expression in 3 osteosarcoma cell lines and extends the current information to demonstrate variability of KIR-ligand transcripts. In this regard, the HOS cell line was found to have nearly absent MHC I (and thus KIR ligand) expression compared to SaOS and U2OS cells. The variability of MHC I and KIR ligand expression underlines the importance of developing a risk-stratification model for further pre-clinical studies with fresh tumor specimens and the subsequent clinical study of KIR incompatible transplantation in pediatric osteosarcoma.
Using the KIR receptor-ligand incompatibility model, we hypothesized that donor NK cells would kill HOS cells (low class I expression) more effectively than both U2OS and SaOS cells (high class I expression). We also expected that SaOS cells would exhibit the least susceptibility to NK mediated killing due to high-level expression of the 3 KIR ligands. As expected, significantly higher lysis of HOS cells was seen when compared to both U2OS and SaOS cells, using NK cells from two different donors. No statistical difference in killing of SaOS and U2OS cells existed suggesting that the difference between 0 and 1 receptor-ligand mismatches may not be functionally relevant. Additionally, early passage SaOS cells were less susceptible to allo-NK cell mediated killing compared to HOS cells as would be predicted using the KIR receptor-ligand incompatibility model because of the substantial differences in MHC class I expression between the two lines. Late passage SaOS cells appeared to lose MHC I and KIR ligand expression, resulting in increased susceptibility to NK-mediated killing.
The potential for alterations in tumor MHC I and KIR ligand expression emphasizes the need to develop better tools for the delineation and monitoring of this critical parameter. Alterations in class I expression have been documented to occur in vivo and may have significant clinically relevant consequences in the response to KIR incompatible transplantation therapies. Additionally, in the setting of conventional therapies in patients with initially high HLA class I and KIR ligand expression, this expression could be monitored throughout treatment. If decreased expression is detected and patients are not responding well to therapy, these patients may benefit from KIR incompatible haploidentical stem cell transplantation as a treatment for relapsed disease.
Few studies have evaluated the effectiveness of NK cell alloreactivity and KIR incompatibility in solid tumors, mostly in the adult population[6
]. One recent publication by Leung et al. evaluated KIR incompatibility and pediatric solid tumors in the conventional setting of autologous stem cell transplantation[9
]. The basis of this prospective study was an earlier finding by this group that a percentage of the population will have self NKs that express an inhibitory KIR(s) without expression of the corresponding ligand(s)[5
]. In that report, the authors found that patients with 2 KIR mismatches had prolonged survival while patients with 1 mismatch or complete KIR compatibility were more likely to develop treatment failures[9
Despite these encouraging results, some autologous transplantation recipients will not have this natural degree of autologous incompatibility and would be more likely to experience a treatment failure. To date, only one paper examining KIR receptor-ligand incompatibility in the setting of haploidentical stem cell transplantation for pediatric solid tumors has been published. In that report, Perez-Martinez et al. describe complete remission and partial response in 2 of 3 children with metastatic solid tumors[10
]. The two patients who responded to treatment demonstrated KIR/KIR ligand mismatch while the non-responding patient was fully KIR/KIR ligand matched[10
]. These findings suggest a graft-vs.-tumor effect toward pediatric solid tumors that may be predicted by KIR/KIR ligand mismatch.
Our study demonstrates that susceptibility of these osteosarcoma cell lines to NK-mediated apoptosis could be predicted based on the degree of KIR receptor-ligand mismatch. We also observed an increased susceptibility to NK-mediated osteosarcoma killing in a cell line that was found to down-regulate MHC class I and KIR ligand transcripts. While this finding strengthened our hypothesis, it does highlight a limitation in using tumor cell lines. Thus, it is necessary to confirm our findings through experiments that down-regulate expression of MHC I and KIR ligand expression (i.e. siRNA) and to perform related experiments in fresh osteosarcoma tumor samples. Following these future experiments, it would be pertinent to test this hypothesis in the clinical setting of haploidentical stem cell transplant for osteosarcoma.