Modulating Metabolic Programming of Memory B cell Activation to Restore B-cell Homeostasis in Autoimmune Disease

In many cell types outside triggers interact with specific proteins on the cells known as receptors to drive the cells to develop and adopt different ways to meet the cellular needs for development. A type of white blood cell known as B cell or B lymphocyte is critical to our immune system. B cell expresses a protein on its surface known as B cell receptor. B cell receptors specifically recognise foreign proteins to promote our immune defense against bacteria and viruses and also in response to vaccination. B cells that have not yet come in contact with a foreign protein are known as naïve B cells and those that had prior exposure are known as memory B cells, which develop into plasma cells that make proteins known as antibodies to target the foreign proteins expressed on bacteria and viruses. However, we do not know how some memory B cells, compared to naïve B cells, are able to respond rapidly to external triggers. A greater proportion of memory B cells are noted in 'Autoimmune Disease' as Rheumatoid Arthritis and Systemic Lupus Erythematosus, also known as Lupus or SLE, particularly during periods of active disease. A better understanding of how B cells use the B cell receptor signal to adapt for successful development may help us to develop treatment to switch off B cell activation without compromising the ability to switch on at times of need, for example, to fight infections.

To this end, it is important to understand how B cells turn the activation signal via the B cell receptor toward successful development into memory B cells and plasma cells. We know that during a state of activation B cells employ specific enzymes to meet the demands on energy and also to generate building blocks for cell development, referred to hereafter as metabolism. In contrast, B cells that are not activated maintain a different program of metabolism. As yet we do not understand the disparity in activation of naïve B cells and memory B cells despite both expressing B cell receptors. Therefore, the proposal seeks to test the hypothesis that the B cell receptors may recruit other proteins to orchestrate different outcomes in response to external triggers.

My preliminary work developed in collaboration with Prof Akbar's group based at University College London suggests that, compared to naïve B cells, memory B cells are more efficient at turning B cell receptor signal into successful metabolic program for cell development. Further, memory B cells express cell surface proteins that co-operate with the B cell receptor. We are able to show these effects in B cells isolated from freshly drawn blood samples from healthy people and also from a small number of people with Lupus. Prof Akbar's group has recently reported that proteins known as sestrins regulate metabolic programming in a closely related type of white blood cell or lymphocyte known as T cell, consequently, the function of a specific type of T cell. However, as yet we do not know the role of sestrins in B cells.

My preliminary data in collaboration with Prof Akbar's group showed that, in blood samples from healthy people, memory B cells that express higher level of proteins similar to antibody secreting plasma cells also had higher activity of enzymes with the potential to serve the cellular demands for energy and building blocks for cell development. Sestrins were also expressed in memory B cells, but not in naïve B cells. We know that sestrins are expressed in response to stress. Therefore, the discrepancy in sestrin expression between naïve and memory B cells is likely related to the metabolic stress or program that memory B cells employ upon activation. We would now like to perform experiments in samples from people with Lupus to understand whether disturbing this pathway may help our efforts to better target B cells to better control the alterations in B-cell profile to improve outcomes for people with autoimmune disease such as Lupus.

A novel BCR-AMPK-Sestrin regulatory pathway of B-cell activation
Upon encounter with specific antigen, the B-cell receptor (BCR) directs naïve B cells to differentiate into memory B cells (MBC) and antibody secreting cells (ASCs). Antigen re-challenge results in disparate response in memory B cells such that some become activated rapidly whereas others remain quiescent. This disparity may be regulated by differential expression of key co-receptors such as CD19 and CD38. Immune defects may alter B-cell homeostasis in autoimmune disease such as Systemic Lupus Erythematosus (SLE).

Further, antigen-challenge evokes a state of cellular stress, which may trigger the expression of stress-inducible proteins, Sestrins. Prof Akbar's group has demonstrated that Sestrins co-operate with AMPK, form a complex with p38 MAPK, Erk and Jnk referred to as Sestrin-p38MAPK complex (sMAC) to drive T cell senescence whilst inhibition of sMAC enhances vaccine responsiveness in old mice. However, their role in B cells is not known.

My preliminary data developed in collaboration with Prof Akbar's team, revealed differential expression of phosphorylated-AMPK (p-AMPK), p38MAPK and sestrins in BCR-stimulated B-cell subpopulations correlating with IgD and CD38 expression in MBC whereas siRNA-mediated knock down of sestrin-2 was associated with significantly lower expression of IgD and CD38. In eight samples from patients with SLE, B cells with higher IgD expression had 2-4 fold higher expression of p-AMPK compared to those with lower IgD expression. Together these results suggest that the BCR recruits CD38 to influence MBC activation.

Therefore, the proposal seeks to explore a novel sestrin-AMPK regulatory pathway of B-cell activation through a series of in vitro experiments using blood samples from healthy people and those with SLE toward Modulating Metabolic Programming of B cells to Restore B-cell Homeostasis in Autoimmune Disease such as SLE.

Benefits for people with autoimmune disease in the long-term
B cells are critical components of our immune system. B-cell hyperactivity is associated with an number of autoimmune diseases such as SLE whereas B-cell dysfunction may lead to immunodeficiency. However, our knowledge of B cell biology in relation to how the B-cell receptor modulates the signal strength to regulate the metabolism remains poorly understood. Currently, B-cells are targeted using monoclonal antibodies (mAbs) either to deplete B cells temporarily using anti-CD20 mAbs such as rituximab or use Belimumab to inhibit their survival. Biological therapies are expensive and repeated B-cell depletion therapy, whilst useful in controlling severe disease activity is also associated with hypogammaglobulinemia and can impair response to immunisation and previously unexposed reflections. Belimumab on the other hand has limited effect on certain B-cell subpopulations. Therefore, a better understanding of B cell signalling and metabolism is critical to make inroads into developing novel B-cell targeted therapies for patients with autoimmune disease in the long-term.

Achieving the objectives set out in the current proposal may unravel a novel-signalling pathway of B-cell activation. A key objective of the proposal is also to understand the role of stress inducible proteins, sestrins, in B-cell activation and function. If our preliminary data is replicated in larger sample then this could have significant implications in the longer term for developing alternative strategies to target B cells to improve outcomes for people with autoimmune diseases such as SLE.

Therefore, in the long-term, research into fundamental aspects of B cell biology is likely to deliver cost-effective B-cell targeted therapies.

Benefits for Scientists with interest in autoimmune disease
B cells play a critical role in the pathogenesis of several autoimmune conditions affecting various organ systems in our body. SLE is a prototype autoimmune disease that can affect several vital organs. Other conditions where B cells make a significant contribution to the pathogenesis include rheumatoid arthritis, ANCA-associated vasculitis, multiple sclerosis and bullous disease of the skin such as pemphigus, where B-cell depletion therapy is effective. Therefore, the proposed research is of relevance to clinicians and scientists interested in improving therapies for people with a range of conditions. Scientists involved in improving vaccine response and immunodeficiency are also likely to benefit from the proposed research about fundamental aspects of B cell biology.

Benefits for molecular and cell biologists
The results of the proposed work may help us understand the interaction between the B-cell receptor and co-receptors. Further, the proposed work probes to understand the consequences of BCR signalling pathway on B cell metabolism. Therefore, knowledge resulting from this work could be exploited to modulate before either to enhance or inhibit B cell function.

Impact on immunologists with interest in senescence
The proposed work would also be of interest to scientists who are already developing methods to target sestrins in the context of senescence.

Therefore, the propose work reaches out beyond the short-term objectives of basic understanding of B cell biology toward long-term objectives of developing novel targets to modulate metabolic programming of memory B cells for the benefit of treating people with autoimmune disease and improving the response to immunisation in people with immunodeficiency.