Resumen de Tumor escape after immunotherapy: implication of hla class i expression in melanoma and bladder tumors
It has been demonstrated that the immune system can recognize tumor cells as non selfelements. This knowledge has prompted the development of different immunotherapies aimed at the passive or active activation of the immune system to eliminate transformed cells. However, cancer immunotherapy clinical trials have shown only partial success, with a large group of patients showing no response to therapy.
Various studies have demonstrated molecular abnormalities in tumor cells that allow them to escape the immune response. One of the major escape mechanisms is alteration in human leukocyte antigen (HLA) class I expression. These alterations could be responsible for the tumor growth in patients with a functional immune system.
In this thesis, we studied different metastases obtained from two melanoma patients showing mixed response after immunotherapy as well as bladder tumors from patients treated with Bacillus Calmette-Guerin (BCG). The first melanoma patient developed several metastases, including 3 progressing and 3 regressing lesions that we obtained and analyzed after autologous vaccination plus BCG (M-VAX). Out of several secondary metastases in the second melanoma patient, we studied 4 regressing and 1 progressing lesions obtained after interferon ¿2b, and 4 regressing and 1 progressing lesions obtained after M-VAX. All metastases showed HLA class I alterations. However, the progressing metastases developed additional and more profound defects in HLA expression.
All metastases from the first melanoma patient presented with LOH in chromosome 6. In addition, progressing metastases showed loss of HLA-B, a weak expression of HLA class I, and LOH in chromosome 15. All metastases from the second melanoma patient had LOH in chromosome 6 and 15 and loss of surface expression of HLA-B. Progressing metastases showed additional defects in the HLA system.
Comparative study of the genome expression pattern in mixed melanoma responders to therapy allowed us to isolate genes differentially expressed in regressing and progressing lesions, with the majority of them being implicated in regulation of the immune response. Upregulation of antigen presentation and immune rejection pathways, including HLA-A, B and C, antigen processing machinery (APM), interferon regulatory factor 1 (IRF-1), signal transducers and activators of transcription 1 (STAT-1), allograft inflammatory factor (AIF-1), granzymes, etc., were found in regressing metastases. These data suggest that regressing tumors are under an acute immune rejection response. The molecular pathways of tumor rejection in our case are similar to those described during allograft rejection, autoimmune disease, graft-versus-host disease and pathogen clearance.
We also studied HLA class I expression in18 bladder cancer patients: 13 of these underwent BCG therapy, with eight relapse-free patients and five patients with relapse after the treatment; and 5 mitomycin-treated patients were used as controls. Post-BCG relapsed bladder tumors had haplotype and/or loci loss before the treatment. In contrast, four relapse-free patients showed no alterations, two showed heterogeneous expression, and two had HLA-B locus loss. We found a higher percentage of structural alterations in lesions with recurrence after BCG: 80% and 60% of tumors showed loss of heterozygosity (LOH) in chromosomes 6 and 15, whereas only 25% of relapse-free patients had LOH in either chromosome.
Bladder tumor relapses showed additional alterations in HLA class I expression in comparison to the primary tumors. Three relapses developed total HLA class I loss and the other two showed an additional loss of HLA-A locus. Mitomycin-treated patients conserve the same HLA class I pattern in primary and relapsed tumor.
Our results show that tumors with additional, irreversible alterations in HLA class I can escape the immune system despite immunotherapy activation. We propose that tumors with reversible soft alterations will upregulate antigen presentation after treatment, leading to recognition and elimination by T cells. In contrast, transformed cells bearing irreversible hard alterations will not show any response after treatment and will continue growing. Analysis of HLA class I alterations in tumor cells is a key factor to select the appropriate immunotherapy.
