Interleukin 21 is a marker of Human African Trypanosomiasis Infection and a contributor to pathology in mice

Background: Human African trypanosomiasis (HAT) is an important disease of sub-Saharan Africa that is approaching elimination in many regions. However, the disease has previously returned from similarly low case numbers in the past, making it important to identify issues that hinder elimination efforts. One important factor is likely to be the recent characterization of individuals with latent HAT infections that are able to tolerate HAT with few symptoms and to control blood parasitaemia to levels that are undetectable by microscopy. Although animal trypanotolerance has been examined in detail, it is unclear how the latent phenotype is maintained in humans. Methods: To identify immune components involved in latent HAT, we used targeted RNASeq to examine the expression of 495 immune-related transcripts in blood collected from 287 individuals at active disease foci in Guinea. These samples included latent infections, HAT clinical cases, and uninfected controls. The in vivo effects of IL21 functional blockade was investigated using a murine model of trypanosomiasis. Results: Differential expression analysis revealed transcripts involved in T cell activation and B cell development that associated with trypanosome infection, including PD1, CD70, and CD80. In particular, IL21 was found to be elevated in infected individuals, although it was significantly higher in clinical cases relative to latent infections. This pattern was replicated at the protein level when patient sera were examined by ELISA. Reducing IL21 pathway activity in mice infected with Trypanosoma brucei led to increased survivorship and reduced parasitaemia in the model animals. Conclusion: Our data show that IL21 is a potential biomarker of Human African Trypanosomiasis and is a cause rather than a consequence of symptoms severity. Further investigation of IL21 will contribute to understanding the factors involved in developing latent HAT, improving control efforts to identify and predict such infections. In the future, the factors identified in this study may also serve as intervention targets to control the symptoms of trypanosomiasis. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement PC, AC, AML were funded by a Wellcome Senior Fellowship to AML (209511/Z/17/Z). BB was funded by IRD. WJK, HN and HI, were supported through the Human Hereditary and Health in Africa (H3Africa) [H3A/18/004]. The second phase of the Wellcome component of H3Africais being implemented by the African Academy of Sciences (AAS) and the NEPAD Agency's Alliance for Accelerating Excellence in Science in Africa (AESA) in partnership with Wellcome. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: All investigations on humans were conducted in accordance with the Declaration of Helsinki. Participants were identified through healthcare providers, community engagement and active surveillance campaigns led by the national contral program, Ministry of Health Guinea. Written informed consents for sample collection, analysis and publication of anonymised data was obtained from all participants by trained local healthcare workers. Subjects or their legal guardian gave consent as a signature or a thumbprint after receiving standardized information in French or their local langage as preferred. Ethical approvals for the study was obtained from within the TrypanoGEN Project following H3Africa Consortium guidelines for informed consent and from Comite Consultatif de Deontologie et ethique (CCDE) at the Institut de recherche pour le Developpent (IRD; 10/06/2013). Research procedures were also approved by the University of Glasgow MVLS Ethics Committee for Non-Clinical Research Involving Human Subjects (Reference no. 200120043). All animal experiments were approved by the University of Glasgow Ethical Review Committee and performed in accordance with the UK Home Office guidelines, UK Animals (Scientific Procedures) Act, 1986 and EU directive 2010/63/EU. All experiments were conducted under SAPO regulations and UK Home Office project licence number PC8C3B25C to Dr. Jean Rodgers. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors and the TrypanoGEN network (H3Africa)