Immune regulation by B cells

Project leader: Prof Marieke van Ham PhD

The regulation of acquired immunity by antigen-specific B cells

This research line addresses the question how the humoral and cellular immune responses are regulated by MHC-mediated antigen presentation in B cells. B cells behave as professional phagocytes of bacteria and particles when recognition is triggered via the specific B cell receptor (BCR). Phagocytosis of Salmonella, our model pathogen, leads to survival of the bacteria in a latent state in the B cells. This is subsequently followed by extracytosis of the bacteria and reinfection in other tissues followed by local multiplication. This year, we have demonstrated in in vivo mouse models that B cells indeed contribute to dissemination of Salmonella in vivo in collaboration with Maria Rescigno (Milan, Italy). Thus, B cells can serve as a niche for survival of Salmonella from the innate immune system and a transport vehicle for systemic dissemination. Additionally, we published data showing that phagocytosis of Salmonella does lead to efficient antigen presentation of bacterial antigens to CD4+ T helper cells and the induction of an efficient humoral immune response is mounted against Salmonella.

In addition, we continued our research on the activation of CD8+ cells by B cells that had taken up Salmonella. Research in animal models has demonstrated that B cells contribute to the cellular immune response against Salmonella, but the mechanism behind it remained unclear. We indicate that efficient cross-presentation of Salmonella antigens by B cells leads to activation of CD8+ memory T cells, but not naive CD8+ T cells. The activation of the recall response induces CD8+ cells that are specifically cytotoxic to Salmonella-infected target cells. Thus, B cells are able to reactivate Salmonella-specific cytotoxic T cells and therefore may contribute to the clearance of Salmonella during reinfection.

The regulation of the humoral immune response by antigen-specific B and CD4+ T cells

We started to investigate how antigens that are taken up by the B cell receptor (BCR) lead to B cell differentiation, antibody formation and, class switching and how this process is aided by activation of specific CD4+ T-helper cell subsets. For this research line, we make use of our infections models and also of hyper-immunization protocols in which patients or donors come in contact with non-infectious antigens on a regular basis. We specifically focus on patients with Rheumatoid arthritis (RA) that are being treated with the therapeutic monoclonal antibody against TNFα: adalimumab. We also focus on voluntary healthy donors that are hyper-immunized against tetanus. A large number of these patients and donors mount an antibody response against the therapeutic antibody or tetanus. For the patients this is detrimental as therapy becomes ineffective because of this inhibitory antibody formation. In collaboration with Genmab (dr. Paul Parren and dr. Janine Schuurman) and Algonomics (dr. Ignace Lasters and dr. Jurgen Pletinckx), we have identified a number of T cell epitopes in the therapeutic antibody adalimumab that may play a role in antibody formation. This year, we have demonstrated actual binding of predicted antigenic peptides to MHC class II by mass spectrometry and we have completed our analyses of T cell reactivity against these epitopes in adalumimab-experienced patients. As expected, T cell reactivity can also be observed in healthy donors that have not been exposed to adalumimab, but at much lower frequencies. For this project we carried on with the generation of MHC class II tetramers as tools to monitor antigen-specific CD4+ T cells. We have expressed monomeric class II complexes associated to CLIP (Class II Associated Invariant Chain Peptide) in insect cells with the aim to generate large production batches of MHCII/CLIP that may be used for the versatile generation of arrays of different MHC II tetramers that each carry the desired antigenic peptide of interest. This year, we focused on the exchange of CLIP for antigenic peptides. The process works in principle, but is inefficient. Therefore, we are currently exploring the use of catalysts of reaction. In addition, we are investigating if we can create expression systems that yield higher levels of MHC/CLIP complexes.

MHC class II-mediated antigen presentation in leukemia as effective MHC class II-Ag presentation is also required for effective cytotoxic T cell function against infected cells or malignant cells. We are studying the regulation of this pathway in leukemia. In B cell chronic lymphocytic leukemia (B-CLL) we demonstrated that aberrant MHC class II antigen presentation is correlated to chronic immune dysfunction of the T cell compartment. In collaboration with dr. Arjan van de Loosdrecht (Dept of Haematology, VUmc), we demonstrated that in leukemic blasts an alternative Ii-independent antigen processing pathway exists that involves TAP-dependent peptide loading of HLA class II complexes. This new route of antigen presentation may affect therapy strategies aimed to optimize antigen presentation in patients.

Key publications