Inhibiting the inhibitors: Checkpoints blockade in solid tumors.

It has been known for many years that the immune system can actively respond to (pre)neoplastic cells and prevent oncogenesis (a concept known as cancer immunosurveillance). However, in most cases, antitumor immune responses are inefficient and neoplasms are allowed to establish. Indeed, effector T cells often infiltrate developing malignant lesions and may significantly influence the prognosis of the disease,, but are frequently functionally impaired. As compared with circulating T cells or T cells infiltrating healthy tissues, these cells (1) secrete reduced amounts of effector cytokines, (2) exhibit limited proliferative capacity, (3) express low levels of pro-survival receptors, and (4) display a terminally differentiated phenotype. Multiple mechanisms have been associated such a functional impairment, most of which can be reconducted to the immunosuppressive nature of the tumor microenvironment. Malignant cells actively secrete cytokines, metabolites and other molecules that directly inhibit the function of antitumor T lymphocytes. In addition, cancer cells can recruit immune cell populations that are capable of hindering effector responses. These include, among others, CD4+CD25+FOXP3+ regulatory T cells and myeloid-derived suppressor cells, which mediate immunosuppressive effects via contact-dependent as well as contact-independent mechanisms. Thus, neoplastic cells as well as immunosuppressive cells of the immune system express multiple inhibitory ligands on their surface that engage cognate receptors on T lymphocytes.

The CD28 family of co-receptors plays a major role in the regulation of immune responses, mostly as they deliver co-stimulatory or co-inhibitory signals to T-cells that have recognized a peptide/MHC complex on the surface of an antigen-presenting cell (APC). Besides CD28, which operates as a major co-stimulatory receptor upon binding to CD80 (also known as B7-H1) or CD86 (also known as B7-H2) on APCs, this family include cytotoxic T lymphocyte-associated protein 4 (CTLA4) and programmed cell death 1 (PDCD1, commonly known as PD-1). CTLA-4, which also interacts with CD80 and CD86, is expressed on the surface of activated T cells (including Tregs) and delivers co-inhibitory signals. Along similar lines, upon interaction with its main ligands (CD274, also known as PD-L1, and PDCD1LG2), PD-1 negatively regulates multiple T-cell functions, including their activation, proliferative potential and ability to secrete effector cytokines. The most prominent physiological role of these molecules is to limit T-cell responses and avoid autoimmune diseases. However, the elevated expression levels of CTLA4 and PD-1 on cancer cells led to the hypothesis that these factors could underlie a major mechanism of immune escape mediated by the inhibition of effector T-cell responses.

Chronic antigenic or inflammatory stimulation induces the expression of CTLA4 and PD-1 on the surface of T cells, as demonstrated in individuals chronically infected with HIV-1 and the hepatitis C virus (HCV). Similarly, these receptors are highly upregulated by tumor antigen-specific T cells, resulting in the robust impairment of effector functions. In line with this notion, the blockade of CTLA4 and PD-1 with specific monoclonal antibodies has been shown to promote tumor regression in multiple preclinical models as it stimulates the function of tumor-specific CD4+ and CD8+ T cells. CTLA4-targeting interventions also boost immune responses by simultaneously interfering with the inhibitory activity of Tregs. These data strongly suggested that the blockade of CTLA4 and/or PD1 could offer a valid approach for the treatment of a wide variety of cancers.

Monoclonal antibodies are nowadays widely employed in anticancer immunotherapy., In 2 clinical trials published in June 2012, 2 different monoclonal antibodies targeting PD-1 or PD-L1 were given to patients affected by different solid tumors, including neoplasms that are generally refractory to immunotherapy such as lung carcinoma., Both antibodies exhibited significant clinical activity as they induced tumor regression (objective response rate of 18–27% and 6–17% for PD-1- and PD-L1-targeting antibodies, respectively) or stabilized disease progression. Importantly, none of the patients with PD-L1- tumors receiving anti-PD-1 antibodies manifested an objective response, suggesting that the expression of PD-L1 on the surface of cancer cells constitutes a robust predictive biomarker that may allow clinicians to implement personalized therapeutic approaches.

In a recent Phase I clinical trial, melanoma patients were treated with the hitherto experimental anti-PD-1 antibody nivolumab plus ipilimumab, a CTLA4-blocking antibody approved by US Food and Drug Administration (FDA) for use in this oncological indication in 2011. The authors reported an objective response rate of 40% and evidence of clinical activity in 65% of the patients. In spite of the fact that up to 53% of patients experienced Grade III-IV adverse events (although reversible, for the vast majority), some degree of tumor regression could be observed in more than 80% of cases.

These results lend strong support to the notion that the dormant immune system can be awaken by the simultaneous inhibition of distinct immunological checkpoints. High-content single-cell analysis of the phenotype and functionality of the immune cell subpopulations involved in such a reactivation will provide further insights into the effector mechanisms that mediate cancer immunosurveillance in humans. Given the large number of functions regulated by CTLA4 and PD-1, these mechanisms are likely to involve multiple cell types. Currently, a number of biological agents including immunostimulatory cytokines such as interleukin-15, are being tested to break the immunological tolerance of cancer patients and boost antitumor T-cell immunity. As it has already been demonstrated in preclinical settings, the combination of immunostimulatory agents with checkpoint inhibitors may boost even further anticancer immune responses and hence constitute an optimal immunotherapeutic approach against (at least some types of) cancer.