Cytokine production by CD4+ T-cells responding to antigen presentation by melanoma cells

Improving antitumor immune responses by circumventing immunoregulatory cells and mechanisms

Although numerous immunotherapeutic strategies have been studied in patients with cancer, consistent induction of clinical responses remains a formidable challenge. Cancer vaccines are often successful at generating elevated numbers of tumor-specific T lymphocytes in peripheral blood, however, despite this, tumors usually continue to grow unabated. Recent evidence suggests that endogenous regulatory cells, known to play a major role in the induction of immune tolerance to self and prevention of autoimmunity, as well as suppressive myeloid cells invoked in the tumor-bearing state, may be largely responsible for preventing effective antitumor immune responses. This review will focus on the major regulatory cell subtypes, including CD4(+)CD25(+) T-regulatory cells, type 1 regulatory T cells, natural killer T cells, and immature myeloid cells. Studies in humans and in animal models have shown a role for all of these cells in tumor progression, although the mechanisms by which they act to suppress immunity remain largely undefined. Elucidation of the dominant molecular mechanisms mediating immune suppression in vivo will allow more precise targeting of the relevant regulatory cell populations, as well as the development of novel strategies and clinical reagents that will directly block molecules that induce the suppression of antitumor immunity.

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Type I Regulatory T Cells in Autoimmunity and Inflammatory Diseases

Regulatory T cells exert a critical role in controlling autoimmunity and maintaining peripheral tolerance. The best described regulatory T cells are the naturally occurring CD4+CD25+ regulatory T cells, which have been shown to be continuously produced within the thymus. Other T-cell subsets bearing suppressive capacity have been reported, including T-helper-3 cells (Th3) and type 1 regulatory T (Tr1) cells. Tr1 cells have been shown to be induced upon antigen exposure under certain tolerogenic conditions and are characterized by the production of the immunosuppressive cytokines IL-10 and TGF-beta, while Th3 cells preferentially produce TGF-beta upon induction by intestinal tolerance. Recent progress has been made in the characterization of Tr1 cells in terms of isolation and induction, respectively. The present review provides an overview of the presence of Tr1 cells in inflammation, infection and neoplastic disorders. Moreover, the relationship between different regulatory T cell subsets and their transcriptional control is discussed. The recent development of methods allowing the ex vivo expansion of regulatory T cells may be the first step towards a cellular therapy with regulatory T cells to control T-cell-mediated pathology in inflammatory disorders.

Regulatory cells and human cancer

While there is now little debate about the existence or relevance of regulatory populations of T cells to a variety of human diseases, including cancer, there is considerable debate about the ontogeny, phenotype, and mechanisms of action of given regulatory T cell populations. This review will limit itself to discussion to two distinct populations of CD4+ regulatory T cells: T cells co-expressing the CD25 receptor (Tregs), and type 1 regulatory (Tr1) T cells. Attention will be focused on the definition and role of these regulatory T cell populations in human cancers.

Immune regulation in tumor-bearing hosts

A wealth of data indicates that tumor immunity directed against a wide variety of malignancies is suppressed in cancer patients. Recent studies have explored the role of 'natural' CD4(+)CD25(+) regulatory T cells (Tregs) in the suppression of tumor immunity in cancer patients. It is now clear, using multiple phenotypic and functional criteria, that the frequency of Tregs is increased in the peripheral blood of cancer patients as well as within the tumor microenvironment. Human Tregs with specificity for tumor-associated antigens have recently been identified, and murine studies have demonstrated that vaccination with tumor-associated antigens can expand Tregs, posing a challenge to cancer vaccine strategies. However, a variety of approaches, including depletion of Tregs or modulation of their activity in vivo, might soon enhance the efficacy of existing cancer vaccines directed against a variety of malignancies.

Dendritic cell density and activation status of tumour-infiltrating lymphocytes in metastatic human melanoma: possible implications for sentinel node metastases

Nodal deposits of melanoma may present many years after resection of the primary tumour, implying initial suppression of tumour growth with subsequent immune escape. Using immunocytochemical techniques on frozen sections, the cellular types and activation status of infiltrating cells within a series of 19 clinically apparent nodal metastases of melanoma were studied. Infiltrating cells were assessed using a semiquantitative grading system. Macrophages (CD68+) and T-lymphocytes (CD3+) (including both CD8+ and probably also CD4+ T-cells) were the predominant cells infiltrating the tumours. B-lymphocytes (CD20+) were generally present in low numbers. CD1a+ putative dendritic cell density and expression of the early lymphocyte activation markers interleukin-2 receptor alpha (IL2Ralpha) and CD69 was low. However, greater evidence of intermediate lymphocyte activation (CD38) was identified. Expression of interleukin-2 (IL2) by tumour-infiltrating cells was not detected. The paucity of staining for IL2 and IL2Ralpha, with greater expression of CD38 by infiltrating cells, suggests that the usual pathways of lymphocyte activation via IL2 were bypassed or impaired within the lymph node metastases. Low numbers of CD1a+ putative dendritic cells may result in reduced effector cell activation. These findings provide evidence to support the hypothesis that antitumour immune responses within clinically involved lymph nodes are reduced in metastatic melanoma. This also has possible implications for micrometastases to the sentinel lymph node.

Significance of plasma cytokine levels in melanoma patients with histologically negative sentinel lymph nodes

INTRODUCTION

Although sentinel lymph node (SLN) status is the most powerful predictor of prognosis in patients with clinically localized melanoma, a proportion of melanoma patients with histologically negative SLNs will still recur. It is hypothesized that tumor response may be altered or mediated by specific cytokines. We therefore investigated whether levels of IL-4, IL-6, IL-10, TNF-alpha, or IFN-gamma would predict disease recurrence in melanoma patients with histologically negative SLNs.

METHODS

This prospective cohort study involved 218 patients with clinically localized melanoma who underwent a histologically negative SLN biopsy. Preoperative plasma cytokine levels were determined by enzyme-linked immunosorbent assay on these patients, as well as on 90 healthy controls. Kaplan-Meier life tables were constructed, and Cox proportional hazards analyses were performed to assess predictors of disease-free survival (DFS).

RESULTS

At a median follow-up of 43 months, 33 of 218 patients (15%) had suffered disease recurrence. Melanoma patients had significant elevations of IL-4, IL-6, and IL-10 compared to healthy controls; levels of IFN-gamma were less elevated in melanoma patients compared to controls. Despite this, melanoma patients with detectable IFN-gamma levels were at significantly higher risk for recurrence compared to patients with undetectable levels (5-year DFS 70% vs. 86%, P = .03). On multivariate analysis including standard melanoma prognostic factors, only tumor thickness (P = .004) and the presence of detectable IFN-gamma levels (P = .05) were significant independent prognostic factors for disease-free survival.

CONCLUSIONS

Among melanoma patients with clinically localized disease who have undergone a histologically negative SLN biopsy, presence of a detectable plasma level of IFN-gamma is an independent predictor of disease recurrence. Elevated levels of IFN-gamma may identify a group of early-stage melanoma patients who are more likely to have recurrence of disease and who may benefit from adjuvant therapies, including immunotherapies.