Since being identified in China in December 2019, coronavirus disease 2019 has rapidly evolved into a global pandemic with over 4 million cases and more than 270,000 deaths.(1) Following the first reported cases in the United Kingdom (UK) in late January 2020, numbers have continued to rise with 223,060 cases and 32,065 deaths reported as of 11 th May 2020.(2) Initial reports from China have indicated that Covid-19 has an overall mortality rate of 1.4%. However, the prognosis varies widely between groups, with age over 60 years and underlying conditions including hypertension, diabetes, cardiovascular disease and cancer identified as risk factors for severe disease and death.(3) The initial reports from China show that patients with cancer are over-represented among individuals who develop severe Covid-19 after contracting the virus.(4) Patients with haematological malignancies are expected to be at increased risk of adverse outcomes from this viral infection, due being immunosuppressed as a consequence of the underlying cancer, and from the effects of therapy. This has led to a variety of recommendations to reduce the risk from Covid-19, including "shielding" by self-isolating at home for prolonged periods, and alterations to therapy such as delaying or even omitting chemotherapy, radiotherapy or transplantation.(5-8) However, at the time of writing there is virtually no published data on the impact of Covid-19 in patients with haematological malignancies.

Summary Patients with haematological malignancies have a high risk of severe infection and death from SARS‐CoV‐2. In this prospective observational study, we investigated the impact of cancer type, disease activity, and treatment in 877 unvaccinated UK patients with SARS‐CoV‐2 infection and active haematological cancer. The primary end‐point was all‐cause mortality. In a multivariate analysis adjusted for age, sex and comorbidities, the highest mortality was in patients with acute leukaemia [odds ratio (OR) = 1·73, 95% confidence interval (CI) 1·1–2·72, P = 0·017] and myeloma (OR 1·3, 95% CI 0·96–1·76, P = 0·08). Having uncontrolled cancer (newly diagnosed awaiting treatment as well as relapsed or progressive disease) was associated with increased mortality risk (OR = 2·45, 95% CI 1·09–5·5, P = 0·03), as was receiving second or beyond line of treatment (OR = 1·7, 95% CI 1·08–2·67, P = 0·023). We found no association between recent cytotoxic chemotherapy or anti‐CD19/anti‐CD20 treatment and increased risk of death within the limitations of the cohort size. Therefore, disease control is an important factor predicting mortality in the context of SARS‐CoV‐2 infection alongside the possible risks of therapies such as cytotoxic treatment or anti‐CD19/anti‐CD20 treatments.

Gastrointestinal (GI) graft-versus-host disease (GvHD) is a major barrier in allogeneic hematopoietic stem-cell transplantation (AHST). The metabolite retinoic acid (RA) potentiates GI-GvHD in mice via alloreactive T-cells expressing the RA-receptor-alpha (RARα), but the role of RA-responsive cells in human GI-GvHD remains undefined. We therefore used conventional and novel sequential immunostaining and flow cytometry to scrutinize RA-responsive T-cells in tissues and blood of AHST patients and characterize the impact of RA on human T-cell alloresponses. Expression of RARα by human mononuclear cells was increased after RA exposure. RARαhi mononuclear cells were increased in GI-GvHD tissue, contained more cellular RA-binding proteins, localized with tissue damage and correlated with GvHD severity and mortality. Using a targeted candidate protein approach we predicted the phenotype of RA-responsive T-cells in the context of increased microenvironmental IL-23. Sequential immunostaining confirmed the presence of a population of RARahi CD8 T-cells with the predicted phenotype, co-expressing the effector T-cell transcription factor T-bet and the IL-23-specific receptor. These cells were increased in GI- but not skin-GvHD tissues and were also selectively expanded in GI-GvHD patient blood. Finally, functional approaches demonstrated RA predominantly increased alloreactive GI-tropic RARahi CD8 effector T-cells, including cells with the phenotype identified in vivo. IL-23-rich conditions potentiated this effect by selectively increasing b7 integrin expression on CD8 effector T-cells and reducing CD4 T-cells with a regulatory cell phenotype. In conclusion we have identified a population of RA-responsive effector T-cells with a distinctive phenotype which are selectively expanded in human GI-GvHD and represent a potential new therapeutic target.

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