The paradox of T-cell-mediated antitumor immunity in spite of poor clinical outcome in human melanoma

Abstract

Human melanoma is hardly ever curable at an advanced stage, but overwhelming evidence from untreated or vaccinated patients indicates that this tumor is highly antigenic and frequently immunogenic. Here, we review recent results indicating that CD8(+) T cell-mediated antitumor immunity is activated at the systemic and tumor level in the early clinical stages (AJCC stages I and II) and continues to be promoted, in a fraction of patients, even in metastatic disease (stages III and IV). This evidence was obtained by looking at frequency, differentiation phenotype, and function of antitumor T cells in periphery and tumor site of melanoma patients. On the other hand, the paradox of immunity in spite of poor clinical evolution of the disease, points toward a model of concurrent evolution of immunity and tumor escape. As melanoma progresses to metastatic disease, powerful mechanisms of tumor evasion from immune recognition, and of immunosuppression, are activated, thus tilting the balance between immunity and escape in favor of tumor resistance to host defense. Nevertheless, recent developments in our understanding of regulation of T cell-mediated immunity can provide clues to the prospects for improved immunotherapy approaches. By integrating the information from basic research in immunology, from murine tumor models, and from trials of immunotherapy, we discuss how the most relevant steps of the antitumor response should be manipulated with greater efficacy by future clinical trials.

Fig. 1A,B

Frequency of cytotoxic T-cell precursors (CTLp) to autologous tumor in 43 metastatic melanoma patients. A LDA was performed as described previously in [34]. This LDA assay does not allow us to detect frequencies of cytolytic T cells <1/200,000. The resulting frequencies for all patients were plotted in a cumulative frequency graph as proportion of patients having cytotoxic cell frequencies equal or higher than a given value in a set of 23 frequency classes. By linear regression analysis, it was estimated that 50% of the patients had either ≤1/5,500 cytotoxic cells against the tumor (a), or ≤1/17,000 CTLp inhibited by anti-CD3 mAb (i.e., all true CTLp) (b), or ≤1/41,500 CTLp inhibited by anti-HLA-class I mAb (i.e., all CTLp restricted to HLA class I antigens) (c). B Comparison of CTLp frequencies in short-term survivors (1–30 months) and long-term survivors (59–151 months) after removal of lymph node metastases, at the time when CTLp had been evaluated; p>0.05 for both CTLp inhibited by anti-CD3 mAb and anti-HLA-class I mAb

Fig. 2A–D

Differentiation phenotype and frequency of MDA- and influenza-specific CD8⁺ T cells in invaded lymph nodes of stage III melanoma patients. A CD8⁺ T cells from tumor-invaded lymph nodes, stained with tetramers to gp100_209–217, Melan-A/MART-1_26–35 and influenza-matrix_58–66, were characterized for the maturation phenotype by four-color flow cytometry, as described in [35]. Proportion of tetramer⁺ T cells showing each of the four possible phenotypes (T _N naïve, T _CM central memory, T _EM effector memory, and T _TD terminally differentiated) defined by staining for CCR7 vs CD45RA is shown with the following color code: 0–10% white, 11–30% yellow, 31–50% light orange, 51–80% red, 81–100% brown. B Frequency of the tetramer⁺ T cells (whose phenotype is shown in A) was expressed as percentage of Tet⁺ cells/CD8⁺ cells. C The same metastatic lesions characterized by tetramer analysis were evaluated for the pattern of T-cell infiltration. To this end, sections were stained, by immunohistochemistry, with mAbs to CD3 and CD8. The results were expressed by the brisk (B), nonbrisk (NB), absent (A) code, as defined in [10]. D Clinical outcome (status and follow-up, in months) of patients after surgical removal of metastatic lymph nodes: NED alive with no evidence of disease, DOD dead of disease, AWD alive with disease

Fig. 3A–C

Expression of HLA class I, HLA-A2, HLA-DR, adhesion molecules, and TAA on 42 human melanoma cell lines isolated from invaded lymph nodes of HLA-A2⁺ patients. Expression of the indicated molecules was evaluated by flow cytometry after staining with specific mAbs [34, 35]. Cell permeabilization was performed to identify expression of Melan-A/MART-1, gp100, tyrosinase, and MAGE-1 in the cytoplasm. The distribution of the antigen expression in the panel of cell lines is described through cumulative frequency plots. In such plots the proportion of tumors showing a fraction of positive cells equal to or higher than an increasing value was plotted against a range of 14 frequency classes ranking from ≥1 to ≥99% positive cells

Fig. 4

A simplified view of immune response to melanoma. Antigen expression and release (step 1) leads to uptake by professional APCs (step 2). After maturation of DCs induced by proinflammatory signals and T-DC interaction, priming of CD4⁺ T cells can take place (step 3). At this stage, a bidirectional interaction between T cells and DCs, mediated by cell-surface regulatory molecules and cytokines, may determine activation versus inhibition of a T cell–mediated response and skewing of a CD4⁺ T cell developmental program toward either a T_H2- (step 6) or a T_H1-type response. Activation of a cell-mediated response is also under the negative regulatory action (−) of suppressor cells (step 4). Priming, clonal expansion, and functional maturation of CD8⁺ T cells (step 5) requires interaction with mature DCs, and with helper T cells. In addition, this step is regulated by several cytokines, which can promote maturation of CD8⁺ T cells to cytolytic effectors, even in an antigen-independent fashion