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Immunotherapy has revolutionized the treatment of cancer. Nevertheless, the majority of patients do not respond to therapy, meaning a deeper understanding of tumor immune evasion strategies is required to boost treatment efficacy. The vast majority of immunotherapy studies have focused on how treatment reinvigorates exhausted CD8+ T cells within the tumor. In contrast, how therapies influence regulatory processes within the draining lymph node is less well studied. In particular, relatively little has been done to examine how tumors may exploit peripheral CD8+ T cell tolerance, an under-studied immune checkpoint that under normal circumstances prevents detrimental autoimmune disease by blocking the initiation of T cell responses. Here we review the therapeutic potential of blocking peripheral CD8+ T cell tolerance for the treatment of cancer. We first comprehensively review what has been learnt about the regulation of CD8+ T cell peripheral tolerance from the non-tumor models in which peripheral tolerance was first defined. We next consider how the tolerant state differs from other states of negative regulation, such as T cell exhaustion and senescence. Finally, we describe how tumors hijack the peripheral tolerance immune checkpoint to prevent anti-tumor immune responses, and argue that disruption of peripheral tolerance may contribute to both the anti-cancer efficacy and autoimmune side-effects of immunotherapy. Overall, we propose that a deeper understanding of peripheral tolerance will ultimately enable the development of more targeted and refined cancer immunotherapy approaches.

The ability of cytotoxic lymphocytes (CL) to eliminate virus-infected or cancerous target cells through the granule exocytosis death pathway is critical to immune homeostasis. Congenital loss of CL function due to bi-allelic mutations in PRF1, UNC13D, STX11, or STXBP2 leads to a potentially fatal immune dysregulation, familial haemophagocytic lymphohistiocytosis (FHL). This occurs due to the failure of CLs to release functional pore-forming protein perforin and, therefore, inability to kill the target cell. Bi-allelic mutations in partner proteins STXBP2 or STX11 impair CL cytotoxicity due to failed docking/fusion of cytotoxic secretory granules with the plasma membrane. One unique feature of STXBP2- and STX11-deficient patient CLs is that their short-term in vitro treatment with a low concentration of IL-2 partially or completely restores natural killer (NK) cell degranulation and cytotoxicity, suggesting the existence of a secondary, yet unknown, pathway for secretory granule exocytosis. In the current report, we studied NK and T-cell function in an individual with late presentation of FHL due to hypomorphic bi-allelic mutations in STXBP2. Intriguingly, in addition to the expected alterations in the STXBP2 and STX11 proteins, we also observed a concomitant significant reduction in the expression of homologous STXBP1 protein and its partner STX1, which had never been implicated in CL function. Further analysis of human NK and T cells demonstrated a functional role for the STXBP1/STX1 axis in NK and CD8+ T-cell cytotoxicity, where it appears to be responsible for as much as 50% of their cytotoxic activity. This discovery suggests a unique and previously unappreciated interplay between STXBP/Munc proteins regulating the same essential granule exocytosis pathway.

Bi-allelic null mutations affecting UNC13D, STXBP2 or STX11 result in defects of lymphocyte cytotoxic degranulation, and commonly cause familial haemophagocytic lymphohistiocytosis (FHL) in early life. Patients with partial loss of function are increasingly being diagnosed after presenting with alternative features of this disease, or with HLH later in life. Here, we studied two sisters with lymphocyte degranulation defects secondary to compound heterozygote missense variants in UNC13D. The older sibling presented aged 11 with linear growth arrest and delayed puberty, two years prior to developing transient ischaemic attacks secondary to neuroinflammation and hypogammaglobulinaemia, but no FHL symptoms. Her geno-identical younge sister was initially asymptomatic but then presented at the same age with severe EBV-driven infectious mononucleosis, which was treated aggressively and did not progress to HLH. The sisters had similar natural killer cell degranulation; however, while cytotoxic activity was moderately reduced in the asymptomatic patient, it was completely absent in both siblings during active disease. Following allogeneic bone marrow transplantation at the age of 15, the older child has completely recovered NK cell cytotoxicity, is asymptomatic and has experienced an exceptional compensatory growth spurt. Her younger sister was also successfully transplanted and is currently disease-free. The current study reveals previously unappreciated manifestations of FHL in patients who inherited hypomorphic gene variants, and also raises the important question of whether a threshold of minimum NK function can be defined that should protect a patient from serious disease manifestations such as HLH.

Congenital perforin deficiency is considered a rare cause of human immunopathology and immune dysregulation, and classically presents as a fatal illness early in infancy. However, we propose that a group of related disorders in which killer lymphocytes deliver only partially active perforin or a reduced quantum of wild-type perforin to the immune synapse should be considered part of an extended syndrome with overlapping but more variable clinical features. Apart from the many rare mutations scattered over the coding sequences, up to 10% of Caucasians carry the severely hypomorphic PRF1 allele C272>T (leading to A91V mutation) and the overall prevalence of the homozygous state for A91V is around 1 in 600 individuals. We therefore postulate that the partial loss of perforin function and its clinical consequences may be more common then currently suspected. An acute clinical presentation is infrequent in A91V heterozygous individuals, but we postulate that the partial loss of perforin function may potentially be manifested in childhood or early adulthood as ‘idiopathic’ inflammatory disease, or through increased cancer susceptibility – either hematological malignancy or multiple, independent primary cancers. We suggest the new term ‘perforinopathy’ to signify the common functional endpoints of all the known consequences of perforin-deficiency and failure to deliver fully functional perforin.