Exploring lymphocyte subsets, autoantibodies and the effect of B cell targeted therapies in rheumatic diseases

Abstract

Nowadays, around 100 different diseases sort under the umbrella of rheumatic diseases(1). They involve different mechanisms and symptoms engaging joints, tendons, ligaments, bones, muscles but also vital organs like the kidneys nervous system and circulation. The diagnoses is dependent on a combination of common clinical manifestations and laboratory findings such as the presence of specific antibodies that target self-antigen (2–4). Rheumatic disease evolves from a complex interplay between genetic, stochastic and environmental factors resulting in a loss of immune tolerance. B-cell targeting agents have proven useful in the treatment of several rheumatic diseases. Rituximab, a chimeric monoclonal antibody targeting CD20, is e.g. used in rheumatoid arthritis (RA) patients who do not respond to tumor necrosis factor (TNF) inhibitors and as off label-rescue treatment in systemic lupus erythematosus (SLE). Additionally, belimumab, a B cell activating factor (BAFF) inhibitor, is the first approved biological drug for SLE. Even though B-cell targeting therapies are commonly used, little is known about the effect of these therapies on disease associated B and T cell subsets, such as age/autoimmune-associated B cells (ABCs) and T follicular helper (Tfh) cells.

In paper I, we therefore studied the effect of these two subsets in response to rituximab treatment in SLE patients and found that ABCs are indeed influenced by the treatment while Tfh frequencies stayed similar to baseline. Additionally, we observed a decrease frequency of programmed cell death protein 1 (PD-1) high CD4+ T cells. In paper II, we investigated the effect of belimumab on circulating B and T cell phenotypes in SLE patients using mass cytometry and correlated them with clinical response. Belimumab had rapid effects on B cell subsets of earlier developmental stages such as naïve B cells while late B cell stages, such as memory or plasma cells, decreased later in a gradual manner or did not change upon treatment. Only early immunological changes correlated with disease improvement. High B cell counts at baseline were associated with late or non-responders. Not all patients respond to B-cell targeting therapy, highlighting the heterogenicity of SLE. To develop novel therapies, a better understanding of the underlying mechanisms related to clinical phenotypes is needed. Thus, in paper III, we explored the cytokine profile and the cellular composition in the synovial fluid of lupus arthritis patients, and we found elevated levels of IL-6 and IL-17A in the joint. Furthermore, we found an enrichment of Th17, peripheral helper T cells and EOMES/Granzyme A-expressing T cells in synovial fluid of SLE patients. All in all, indicating a potential role of Th17 cells in the pathogenesis of lupus. In contrast to lupus arthritis, RA patients exhibit a more aggressive form of arthritis, especially in conjunction with anti-citrullinated protein autoantibodies (ACPA). Therefore, in paper IV, we investigated the citrulline-specific B cell population in RA patients using an antigen-tetramer enrichment technology followed by single cell sequencing of the immunoglobulin genes. We discovered that the broad ACPA specificity in RA patients might develop from clonal expansion of a few B cell clones. Furthermore, monoclonal antibodies which originated from citrulline reactive B cells were multi-reactive and able to promote pain-like behavior and joint inflammation in mice.

Overall, this thesis explored immunological changes upon B-cell-targeting therapy and pathogenicity in joint inflammation in SLE and seropositive RA patients. We provided new understanding of B-cell targeting therapies on B and T cell subsets in SLE patients as well as the pathogenic mechanism which might be involved in lupus arthritis. Additionally, we examined the citrulline reactive B cell repertoire in RA patients and our data reveal their potential origin.