Tumor escape mechanisms: potential role of soluble HLA antigens and NK cells activating ligands

Abstract

The crucial role played by human leukocyte antigen (HLA) antigens and natural killer (NK)-cell-activating ligands in the interactions of malignant cells with components of the host's immune system has stimulated interest in the characterization of their expression by malignant cells. Convincing evidence generated by the immunohistochemical staining of surgically removed malignant lesions with monoclonal antibodies recognizing HLA antigens and NK-cell-activating ligands indicates that the surface expression of these molecules is frequently altered on malignant cells. These changes appear to have clinical significance because in some types of malignant disease they are associated with the histopathological characteristics of the lesions as well as with disease-free interval and survival. These associations have been suggested to reflect the effect of HLA antigen and NK-cell-activating ligand abnormalities on the interactions of tumor cells with antigen-specific cytotoxic T lymphocytes (CTL) and with NK cells. Nevertheless, there are examples in which disease progresses in the face of appropriate HLA antigen and/or NK-cell-activating ligand as well as tumor antigen expression by malignant cells and of functional antigen-specific CTL in the investigated patient. In such scenarios, it is likely that the tumor microenvironment is unfavorable for CTL and NK cell activity and contributes to tumor immune escape. Many distinct escape mechanisms have been shown to protect malignant cells from immune recognition and destruction in the tumor microenvironment. In this article, following the description of the structural and functional characteristics of soluble HLA antigens and NK-cell-activating ligands, we will review changes in their serum level in malignant disease and discuss their potential role in the escape mechanisms used by tumor cells to avoid recognition and destruction.

Figure 1. Molecular mechanisms underlying the functional properties of HLA and NK cell activating ligand expression by malignant cells

(A) Once transported to the plasma membrane, the classical HLA class I-β₂m-peptide complex plays a major role in the interactions between target cells and (a) activation of peptide-specific CTL through TCR; and (b) inhibition of T cell subpopulations through inhibitory receptors KIR. (B) In contrast to classical HLA class I, the non-classical HLA class I, HLA-G, inhibits CTL, CD4(+) T cells and NK cells through its interaction with the NK cells receptor CD94/NKG2. (C) MICA/B as well as ULBP ligand expression by tumor cells may be potentially beneficial to TA-specific immune responses through their interaction with the NK cells receptor CD94/NKG2 on NK cells and subsets of T cells, resulting in the activation of NK and T cell-mediated killing.

Figure 2. Structural properties of soluble HLA antigens

(A) HLA class I heavy chains () are detectable in plasma as 43kDa, 39kDa and 35kDa moieties which represent membrane associated, alternatively spliced and metalloprotease cleaved forms, respectively. Splicing of exon 5 results in the removal of amino acids depicted in white (●), but not residues encoded by exons 6 and 7 (●). Metalloprotease mediated cleavage likely results in removal of amino acids encoded by exons 5–7. All three forms are shown to be associated with β2m () and peptide (); however, β2m-free heavy chains have also been detected. (B) HLA-G is detectable in seven forms. Four of them, HLA-G1, -G2, -G3 and -G4, are bound to the cell surface, while the remaining three, HLA-G5, -G6 and -G7 are soluble. HLA-G1 is the only isoform derived from the translation of the total HLA-G transcript. The other membrane bound isoforms lack one or two globular domains. The structure of the soluble isoforms resembles that of the corresponding membrane bound isoforms in the extracellular part, but differs at the C-terminus. The extracellular domain and the intracytoplasmic tail, which are present in the membrane bound isoforms, are replaced in the secreted isoforms by a short hydrophilic tail. These differences provide a marker to distinguish shed or proteolytically cleaved HLA-G isoforms from secreted HLA-G isoforms.

Figure 3. Immune escape mechanisms utilized by tumor cells

Escape mechanisms utilized by tumor cells include: i) HLA class I antigen-TA derived peptide complex loss which can result from loss of a) TA, b) APM antigen processing machinery components, or c) HLA class I antigens;ii) release of immune suppressive small molecules such as PGE2, INOS and/or H₂O₂; iii) release of immune suppressive cytokines resulting in altered immune cell function; iv) Fas ligand expression resulting in the killing of Fas+ lymphocytes; g) over-expression of anti-apoptotic proteins in melanoma cells resulting in apopotitic resistance; and v) expression of tumor associated gangliosides which can inhibit IL-2 dependent lymphocyte proliferation as well as induce apoptotic signals.

Figure 4. Potential role of sHLA and sNKAL in the escape of tumor cells from the host's immune response

The immune system can target tumor cell growth through several mechanisms. (A) It is thought that the most effective way of mounting a TA-specific immune response is through the combined action of CD8(+) and IFN-γ-secreting CD4(+) T helper cells (Th1). TA-specific CD8(+) T cells can be activated by antigen presenting cells (APC) and kill tumors cells directly. The survival and persistence of CD8(+) T cells is dependent upon CD4(+) T helper cells. Naïve CD4(+) Th1 cells recognize HLA class II antigen-peptide complexes, through their T cell receptor (TCR), on the surface of APC. This interaction leads to the generation of i) Th1 helper cells which promote survival and proliferation of CD8(+) T cells and ii) cytotoxic CD4(+) T cells, which can directly kill HLA class II antigen expressing tumor cells. In addition, both CD8(+) and CD4(+) T cells secrete IFN- γ, which can further sensitize tumor cells to CD8(+) T cell-mediated killing by upregulating HLA class I antigens and APM components, promoting the recruitment of natural killer (NK) cells, granulocytes and macrophages, as well as inhibiting angiogenesis within tumor stroma. Tumor growth can also be controlled by IL-5-secreting CD4(+) T helper cells (Th2). APC activate IL-5 Th2 cells, which induce the accumulation of eosinophils and/or provide help for the generation of an antibody-based TA-specific immune response. NK cells might also play a role by recognizing ‘stress’ or ‘danger’ signals that are produced by tumors. Once activated NK cells may contribute to the tumor immune response through (i) direct lysis of tumor cells, (ii) indirectly providing TA to APC for presentation to CTL, (iii) activating CD4+ T and B-cells as well as CTL through the secretion of cytokines such as IFN- γ. (B) The release of soluble classical and non-classical HLA antigens as well as NK cell activating ligands may lead to downregulation of the host's TA-specific immune response through (i) induction of apoptosis of both CTL as well as NK cells, (ii) indirect downregulation of CD4+ T helper as well as B cell reponses via suppression of IFN- γ secondary to NK cell apoptosis, and (iii) altering the level of membrane bound HLA antigen and NKAL ligand expression thereby hindering their recognition by T and NK cells, respectively.