Inhibiting the inhibitors

Keywords: :

• cancer immunotherapy
• CTLA4
• immune checkpoints
• monoclonal antibodies
• PD-1
• T cells

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).¹ 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,^2,3 but are frequently functionally impaired.⁴ 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.⁵ 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.⁵ 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).⁶ 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.⁶ 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.⁷

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).⁸ 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.⁷ CTLA4-targeting interventions also boost immune responses by simultaneously interfering with the inhibitory activity of Tregs.⁷ 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.⁹

Monoclonal antibodies are nowadays widely employed in anticancer immunotherapy.^10,11 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.^12,13 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.¹³

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 2011.¹⁴ 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-15,¹⁵ 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,¹⁶ 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.

Abbreviations:
APC	=	antigen-presenting cell
CTLA4	=	cytotoxic T lymphocyte-associated protein 4
FDA	=	US Food and Drug Administration
HCV	=	hepatitis C virus
PD-1	=	programmed cell death 1
Treg	=	regulatory T cell

Citation: Mavilio D, Lugli E. Inhibiting the inhibitors: Checkpoints blockade in solid tumors. OncoImmunology 2013; 2:e26535; 10.4161/onci.26535