The human major histocompatibility complex (MHC) HLA codes polymorphic cell surface proteins that are grouped into two main categories: class I and II [1]. HLA-class-I proteins are composed of a heavy-chain and the β2-microglobulin light chain that are assembled in the endoplasmic reticulum with the contribution of the transporter associated with antigen processing proteins (TAPs) that bring together the chains with antigenic peptides. HLA-class-I expression is essential for T-cell mediated cytotoxicity against cells expressing foreign peptides, like non-self cells or virally infected cells. All nucleated cells express HLA-class-I except for trophoblast.

Immunotherapeutic approaches exploiting T-cell antitumor activity require the presence of cancer-specific antigens on cancer cells that can be recognized by immune cells. T cell-based assays have identified antigens from various tumor cells and HLA class I-restricted epitopes, including Her2- and mamaglobulin-derived peptides in BCa [2], [3]. Loss of HLA-class-I antigens, however, occurs in cancer cells, allowing escape from T-cell surveillance. In 1995, we reported that one-third of breast carcinomas suffer from an orchestrated loss of HLA-class-I, β2-microglobulin, and TAP1 expression, thus loss of all components necessary for antigen presentation [4]. Multiple mechanisms of HLA-loss have been identified, including HLA- and β2-microglobulin gene mutations, oncogene activation, mutations of the interferon pathway-related genes, and a variety of post-translational silencing mechanisms [5]. Moreover, cancer stem cells have been shown to evade the immune system's attack by suppressing HLA-class-I molecule expression [6], [7].

Loss of HLA-class-I expression is important in the outcome of immunotherapy with immune checkpoint inhibitors (ICIs). Zhang et al. showed that loss of HLA heterozygosity in lung cancer patients is linked with poor response to ICIs [8]. High expression of HLA-class-I in a series of 146 patients with metastatic non-small- cell lung cancer treated with ICIs defined better clinical outcomes [9]. Loss of HLA–I heterozygosity has been also linked with poor outcome of melanoma patients treated with ICIs [10].

The tumor microenvironment (TME) also has an important role in antitumor immunity, as it can suppress cytotoxic responses even in the presence of antigenic presentation by cancer cells. Hypoxia promotes immunosuppression by facilitating the prevalence of regulatory T-cells and M2-type myeloid cells in the TME [11]. Disruption of hypoxia with pharmacological targeting reduces myeloid cells in experimental tumors and sensitizes cancer cells to PD-1 blockade [12]. Moreover, lactate release from cancer cells under hypoxic conditions promotes M2-polarization, VEGF release by tumor-associated macrophages, and blocks T-cell proliferation [13], [14]. Blockage of LDH promotes CD8 + cytotoxic cell immunity [15]. Whether hypoxia also affects the expression of HLA-class-I molecule is unclear. We previously reported that anaerobic metabolism is linked with loss of HLA-class-I expression in lung cancer, and exposure to hypoxia suppresses its expression in cell line experiments [16].

In the current study, we investigated the expression patterns of HLA-class-I molecules in a series of human breast carcinomas and the association of HLA-class-I expression loss with histopathological variables and prognosis. Moreover, we provide evidence that microenvironmental conditions, like hypoxia and tumor-infiltrating lymphocytes and IFNγ, play an important role in HLA-class-I expression regulation and, eventually, in immune surveillance.