Human Leukocyte Antigens (HLAs), HLA-DP among them, are crucial components of the immune system. Their role is to present immunogenic peptides to T cells, triggering an immune response. This can be protective if the target antigens originate from infecting pathogens, but it can also cause unwanted immune reactions if the antigen is of self-origin, such as in the case of autoimmune diseases or many complications associated with allogeneic stem cell transplantation. Understanding how certain antigens are presented by the HLA molecules offers the possibility to prevent or treat several clinical conditions. HLA-DM in this setting plays a key role for HLA-DP and other similar molecules. It acts as a sort of “molecular filter” selecting only for certain antigens for presentation, therefore influencing the ability of HLA-DP and specific antigens to trigger T cell immune responses.

It was a great honor for me and my working group, and an important appreciation of our work and ideas. Every day we challenge ourselves, in the attempt to better understand human biology, in particular the mechanisms regulating the immune system and how this can be quickly translated into patients’ benefit. This is certainly very exciting, but can also be frustrating from time to time. Receiving the DKMS John Hansen Research Grant was, above all, an important boost of enthusiasm and motivation for our work.

This was the first grant I ever received and it proved to be greatly valuable for my career development. The results we achieved contributed to improving my profile within our research field and supported my integration and networking within the University Hospital Essen and beyond. In particular, the data and ideas generated through this grant enabled us to formulate and test hypotheses in the clinical settings with the support of major international registries in the field of stem cell transplantation such as the Center for International Blood and Marrow Transplant Research (CIBMTR) and the European Society for Blood and Marrow Transplantation (EBMT).

The financial support was perfectly fitting to our initial experimental plan, and allowed us to even explore the use of new technologies and establish new experimental models both for the investigation of other HLA antigens, as well as exploring HLA antigen presentation in other contexts, such as immunity to cytomegalovirus.

The most important outcome we achieved with this grant was demonstrating that the antigen repertoire presented by the HLA molecules of patients and donors – commonly referred to as the immunopeptidome – plays a crucial role in determining the potential benefits and drawbacks of stem cell transplantation. In particular, we found that the degree of divergence between the immunopeptidomes of patients and donors can be used to predict clinical transplantation outcomes, potentially guiding a more informed and strategic selection of donors prior to transplantation.

This finding represents the latest advancement in the concept of intelligent donor selection, initially proposed and developed for HLA-DP by my mentor, Prof. Dr. Katharina Fleischhauer. Her work introduced the use of T cell epitope (TCE) groups based on functional testing of alloreactive T cells as an approach to identify more or less tolerated HLA-DP mismatches. This approach proved to be successful in predicting clinical outcome. Later, TCE groups were shown to be a reflection of the immunopeptidome divergence between the HLA-DP of patients and donors, with a strong contribution by the editing activity of HLA-DM. Within the DKMS John Hansen Research Grant we further explored this observation, expanding it to a large collection of HLA-DP types and confirming that immunopeptidome divergence and HLA-DM editing control alloreactivity elicited by different HLA-DP mismatches. We further extended this mechanism to HLA class I antigens defining functional groups based on peptide binding motifs (PBM) derived from immunopeptidome analysis. We showed that this classification model for HLA class I mismatches can be used to inform the risk associated with stem cell transplantation.

This is partially answered above. Additional considerations arise from the recent developments in the clinical practice, including new strategies to prevent transplant toxicity and graft-versus-host disease (GvHD), such as the use of post-transplant cyclophosphamide (PTCy). These strategies are producing important changes, in particular regarding the role of HLA in stem cell transplantation.

Before PTCy, compatibility between patients and donors was determined by genetic identity between the main HLA loci of patients and donors, limiting the access to transplantation only to patients with HLA matched donors. With PTCy, HLA mismatches are no longer a barrier to stem cell transplantation. Instead, they may represent an opportunity, especially in the treatment of blood malignancies, as they can be exploited to enhance the efficacy of transplantation in curing leukemia.

Our work, digging into the molecular mechanism driving immune reactions to HLA mismatches, aims to improve our understanding of HLA antigen presentation and how this can be modulated to exploit HLA mismatches and improve stem cell transplantation in the future.

Our work focused on understanding the role of HLA mismatches in stem cell transplantation and how they can either elicit toxic graft-versus-host disease or be possibly exploited for a protective graft-versus-leukemia effect (GvL). The goal is to identify specific HLA mismatches between patients and donors that offer the optimal balance between reducing the risk of toxic GvHD while preserving, if possible, the beneficial GvL effect mediated by T cell alloreactivity.

The DKMS John Hansen Research Grant supported us particularly in implementing an experimental pipeline for the characterization of HLA immunopeptidomes. It also enhanced our knowledge of this method, so that we could employ it not only for HLA-DP but also for other HLA antigens. Without the grant, the progress of our research would have been considerably more limited.

The DKMS John Hansen Research Grant was crucial in further developing our initial collaboration with Prof. Barbara Sitek from the Medical Proteome Center of the Ruhr University Bochum. Our data were presented at different conferences organized by prominent societies in the field of stem cell transplantation, such as European Federation of Immunogenetics (EFI) and EBMT. It allowed us to get in contact with the immunopeptidomics community (the Human Proteome Organization – Human ImmunoPeptidome Project, HUPO-HIPP), getting the chance to start collaborations with other researchers in this field as Prof. Christian Freund and Dr. Miguel Álvaro-Benito from the Free University of Berlin.

The DKMS John Hansen Research Grant represents an important opportunity for young researchers to establish their career and develop independent research, especially in such a highly competitive field. For me, it was of great value and support.

Pursuing research requires strong passion and dedication, as well as resilience in facing negative results or critical feedback. My advice is to actively engage with colleagues and mentors, as they can serve as a first line of reviewers to our work: providing insights and helping to improve and refine new ideas.

I am currently continuing the same line of research, focusing on immunopeptidome divergence and its modulation by HLA-DM and other components of the HLA class II machinery for HLA-DR and HLA-DQ mismatches. Considering that these mismatches are expected to become more frequent with the introduction of PTCy, I believe this type of research has the potential to inform and improve future clinical practice, possibly guiding a strategic donor selection.

Looking ahead, it is exciting to speculate about strategies to therapeutically modulate HLA immunopeptidomes with the goal of making malignant diseases more immunogenic. While these approaches are for the moment far from being applicable, our growing understanding of HLA antigen presentation and T cell alloreactivity on a broader scale - beyond the single HLA locus - might help determine whether such strategies could be pursued in the future.

The DKMS John Hansen Research Grant has definitely had a significant impact on my career, supporting me during the critical phase after my doctoral degree. It helped me integrate the fields of immunogenetics and proteomics into my daily research activity, demonstrating how important their combination - immunopeptidomics – is for investigating and understanding T cell immunity.