Article written by Vera Siffnerova (FAS, Omixon)
In most Slavic languages, LOH stands for Summer Olympic Games (e. g. Letní Olympijské Hry in Czech), in HLA world, LOH is known as an abbreviation of an important disease marker – Loss of Heterozygosity – during which different types of cancer cells try to escape immunity mechanisms to win their own “survival game“.
Looking at this phenomenon from a wider perspective, loss or down-regulation of HLA Class I antigens in tumor cells has been frequently observed in a variety of human malignancies(1-5). The first description of MHC Class I loss was done in a mouse model (Gardener lymphoma) by Dr. Festenstein in 1976(6). This mechanism is not an exclusivity of cancer cells, but also of viruses. It has been found that viruses use a similar mechanism to avoid recognition and elimination by the immune system(7).
Recent advances in HLA haploidentical hematopoietic stem cell transplantation (haplo-HSCT) have been associated with significant reduction in treatment-related mortality. However, the most common cause of treatment failure, as with any other type of allogeneic transplant, remains to be the disease relapse. While, in general, etiology of relapses after haploidentical transplantation may overlap with other types of allogeneic transplants, a unique relapse mechanism with loss of heterozygosity is only observed after haplo-identical transplantation. This kind of transplantations may offer augmented graft-versus-leukemia (GvL) effect owing to its unique treatment platform, HLA disparity and, in certain situations, enhanced natural killer (NK) cell alloreactivity. Preemptive infusions of modified donor lymphocytes, NK cells, or chimeric antigen receptor (CAR) T-cells after haploidentical transplantation are interesting areas of ongoing research to prevent and treat relapses(9).
LOH as a possible mechanism of leukemia immune escape after haplo-HCT has been described by several groups. Vago and coworkers have studied the genomic rearrangements in mutant variants of leukemia by using genomic HLA typing, microsatellite mapping, and single nucleotide polymorphism (SNP) arrays. They identified the specific mutant variant of leukemia cells with HLA that differed from the donor’s haplotype which had been lost because of acquired uniparental disomy (aUPD) of chromosome 6p in 5 of 17 patients with leukemia relapse after haploidentical transplantation. This mutation resulted in leukemic cell evasion from donor T-cell recognition, whereas the original leukemic cells taken at the time of diagnosis were efficiently recognized and killed. The authors hypothesized that HLA loss may reflect alloimmune pressure mediated by donor T-cells toward the HLA mismatches(10).
This phenomenon was also confirmed in a report by Villalobos and coworkers, describing two cases of HLA loss of chromosome 6p aUPD that resulted in a total loss of the mismatched HLA haplotype among three pediatric patients with AML who relapsed after haplo-HSCT. The molecular events that form the basis of this type of genomic abnormality remain uncertain, but it has been postulated that aUPD may derive either from mitotic homologous recombination events or from an attempt to correct for the unbalanced loss of chromosomal material by using the remaining alleles as a template resulting in copy number neutral-LOH without a concurrent change in the copy number; therefore, standard cytogenetic methods fail to detect this phenomenon(12).
Another mechanism of LOH has been described by McCurdy et al in a study of two high-risk AML patients who relapsed after haplo-HCT using posttransplant cyclophosphamide. In this report, the authors demonstrated the absence of a mismatched recipient HLA haplotype on the isolated leukemic blasts in both cases. Interestingly, both cases represent distinct mechanisms of HLA loss. SNP array for recipient 1 demonstrated aUPD at 6p, which is the same mechanism that was previously described. However, the karyotype and SNP array for recipient 2 revealed a deletion of chromosome 6p that encompassed the mismatched HLA locus. The later represents a different, but similar, genomic mechanism and supports that the leukemic cells may lose the mismatched HLA haplotype through multiple means, resulting in evasion of the donor immune system(13).
The last example of mechanism causing down-regulation of mismatched HLA class I antigens was described by Tamaki et al. In this study, this group found a lack of mismatched HLA-A using flow cytometric analysis, despite the retention of both HLA haplotypes on the leukemic cell surface of the AML patient who relapsed after haplo-HSCT. They speculated that this finding might be associated with impaired epigenetic regulation of the gene causing down-regulation of HLA class I on unshared alleles, which are preserved on shared alleles(14).
It becomes crucial to document the HLA loss in patients who have leukemia relapse after haplo-HSCT because it has several relevant clinical consequences. Not only does it demonstrate that the donor-derived T-cells circulating in the patient at the time of relapse become inefficient bystanders, but also that any attempt to induce remission by infusion of donor T-lymphocytes is expected to be ineffective against the leukemic cells and potentially harmful to the patient due to the conserved risk of inducing GvHD(15).
Based on the net result of the genomic alteration, two possible alternative immunotherapeutic strategies can be considered for these variants of leukemia relapse. The first is a second transplantation from a different HLA-haploidentical donor, selected for being mismatched against the HLA haplotype retained by leukemic blasts. Theoretically, the advantage of this method is that the donor and leukemia cells would have a full immunologic incompatibility that can help increase GvL effect, while an incompatibility between the donor and patient’s healthy tissues is only 50%. The second treatment option is an infusion of high-dose purified donor NK cells. It is based on an observation that leukemic cells that undergo genomic loss of one HLA haplotype in several cases also lose the ligands for donor inhibitory KIRs, becoming in principle susceptible to NK cell alloreactivity. Even though, the effectiveness of this strategy is limited in overt leukemia relapse, it might help in preemptive treatment of impending leukemia recurrence, guided by molecular markers of minimal residual disease and early detection of HLA loss relapse(16).
Haplo-HSCT has evolved from a high-risk procedure to a widely acceptable alternative donor transplant option with similar toxicity and outcomes compared to HLA-matched donor transplants in retrospective studies. Loss of heterozygosity aspects and related studies results have been successfully used to improve transplantation outcomes. Ongoing continuous investigations on this topic still bring more questions and some uncertainties (similar to any other research). Once answered or understood, LOH affairs can possibly be nominated as a new top sport at HLA World Olympic transplant playground.
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