Despite the success of monotherapies based on blockade of programmed cell death 1 (PD-1) in human melanoma, most patients do not experience durable clinical benefit. Pre-existing T-cell infiltration and/or the presence of PD-L1 in tumours may be used as indicators of clinical response; however, blood-based profiling to understand the mechanisms of PD-1 blockade has not been widely explored. Here we use immune profiling of peripheral blood from patients with stage IV melanoma before and after treatment with the PD-1-targeting antibody pembrolizumab and identify pharmacodynamic changes in circulating exhausted-phenotype CD8 T cells (Tex cells). Most of the patients demonstrated an immunological response to pembrolizumab. Clinical failure in many patients was not solely due to an inability to induce immune reinvigoration, but rather resulted from an imbalance between T-cell reinvigoration and tumour burden. The magnitude of reinvigoration of circulating Tex cells determined in relation to pretreatment tumour burden correlated with clinical response. By focused profiling of a mechanistically relevant circulating T-cell subpopulation calibrated to pretreatment disease burden, we identify a clinically accessible potential on-treatment predictor of response to PD-1 blockade.

SUMMARY Exhausted CD8+ T cells (TEX) in chronic infections and cancer have limited effector function, high inhibitory receptor co-expression and extensive transcriptional changes compared to effector (TEFF) or memory (TMEM) CD8+ T cells. TEX are important clinical targets of checkpoint blockade and other immunotherapies. Epigenetically, TEX are a distinct immune subset, with a unique chromatin landscape compared to TEFF and TMEM. However, the mechanisms governing the transcriptional and epigenetic development of TEX remain unknown. Here, we identify the HMG-box transcription factor TOX as a central regulator of TEX. TOX is largely dispensable for TEFF and TMEM formation, but is critical for exhaustion and without TOX TEX do not form. TOX is induced by calcineurin and NFAT2 and operates in a feed-forward loop to become calcineurin independent and sustained in TEX. Thus, robust TOX expression results in commitment to TEX by translating persistent stimulation into a distinct TEX transcriptional and epigenetic developmental program.

Summary Antibodies capable of neutralizing HIV-1 often target variable regions 1 and 2 (V1V2) of the HIV-1 envelope, but the mechanism of their elicitation has been unclear. Here we define the developmental pathway by which such antibodies are generated and acquire the requisite molecular characteristics for neutralization. Twelve somatically related neutralizing antibodies (CAP256-VRC26.01-12) were isolated from CAPRISA-donor CAP256; each antibody contained the protruding tyrosine-sulfated, anionic antigen-binding loop (CDR H3) characteristic of this category of antibodies. Their unmutated ancestor emerged between weeks 30–38 post-infection with a 35-residue CDR H3, and neutralized the virus that superinfected this individual 15 weeks after initial infection. Improved neutralization breadth occurred by week 59 with modest affinity maturation, and was preceded by extensive diversification of the virus population. HIV-1 V1V2-directed neutralizing antibodies can thus develop relatively rapidly through initial selection of B cells with a long CDR H3, and limited subsequent somatic hypermutation, an important vaccine insight.