BTK Signaling and Implications for CSU

Welcome to our presentation on Bruton's tyrosine kinase signaling and its implications on chronic spontaneous urticaria. I am Jonathan Rodrigues, a Medical Director with Dermatology and Allergy at Novartis Pharmaceuticals.

And I'm Merin Kalangara, also a Dermatology and Allergy Medical Director with Novartis Pharmaceuticals.

Our objectives for today are to have an overview of the current understanding of CSU immunopathology and to understand the rationale for the potential therapeutic targeting of BTK in CSU.

While the pathogenesis of CSU is not fully understood, autoimmunity is thought to be a driving factor.

This involves IgE and IgG autoantibodies in types I and IIb CSU endotypes, respectively.

Both of these pathways converge on the high-affinity IgE receptor - or Fc epsilon receptor I – on the surface of mast cells and basophils, setting off an intracellular signaling cascade that relies on several mediators, including BTK – or Bruton's tyrosine kinase – for cellular activation and degranulation, leading to histamine release.

Mast cells also release cytokines and chemokines responsible for recruiting a perivascular cellular infiltrate that includes eosinophils, basophils, and T cells.

While B cells are not part of this cellular infiltrate, they contribute to CSU pathophysiology through the generation of these autoreactive IgE and IgG antibodies.

In type I – or autoallergic – CSU, specific IgE is generated against various self-antigens – or autoallergens, and this complex then binds to the high-affinity IgE receptor on the mast cell surface, initiating an intracellular signaling cascade dependent on several mediators, including BTK, for activation and degranulation.

In the first subendotype of type IIb autoimmune CSU, IgG autoantibodies are detected against the high-affinity IgE receptor itself, again resulting in receptor crosslinking and mast cell activation that is BTK-dependent.

And finally, the second subendotype of type IIb autoimmune CSU is mediated by IgG autoantibodies that are directed against the IgE molecule.

Regardless of the underlying mechanism, all of these pathways converge on the high-affinity IgE receptor and rely on BTK for mast cell activation and degranulation.

Highlighting the role of BTK in CSU, in one recent study, a systems biology-based computational model was used to create a CSU interactome that was validated with gene expression data from patients with CSU.

This model emphasized the central role of mast cell activation, as well as the importance of the IgE–high-affinity IgE receptor axis.

Specifically, looking at the top 20 CSU related proteins involved in mast cell activation, the high-affinity IgE receptor was number 2, and BTK was number 4.

The same study also presented this multidimensional scaling representation of a mast cell activation protein network and modulation induced by different targets.

The representation shows the area of action of each target in general, as represented by the bluish shadow, and specifically for mast cell activation and degranulation, as seen by the greenish shadow, as well as the strength of this impact shown in this T signal scale on the right side.

And as you can see, BTK has an obviously strong relationship with mast cell activation and degranulation.

BTK is a non-receptor cytoplasmic TEC tyrosine kinase protein.

It is a multicomponent signaling protein with adaptor and kinase functions.

It is expressed in many hematopoietic cell types, including B cells, mast cells, and myeloid cells.

BTK regulates high-affinity immunoglobulin IgE receptor I signaling in mast cells, and this function could position it as an attractive molecule in IgE-mediated diseases.

BTK integrates B-cell receptor signaling to regulate B-cell development.

IgE antibodies bind Fc epsilon receptors on mast cells and basophils to trigger degranulation and acute inflammation, whereas IgG binds high-affinity immunoglobulin G receptor on macrophages, plasmacytoid dendritic cells, and natural killer cells to promote cellular activation or phagocytosis.

BTK has low activity in resting cells, but its activity increases 10- to 20-fold in stimulated cells.

Phosphorylation of BTK at the tyrosine 223 and 551 residues leads to phospholipase C gamma 2 activation, which further induces the activation of transcription factors such as nuclear factor of activated T cells, nuclear factor kappa B, and mitogen-activated protein kinase, which lead to cellular activation, proliferation, and differentiation, and the release of cytokines and other inflammatory mediators.

BTK is activated in mast cells upon crosslinking of high-affinity IgE receptor I. It is the central positive regulator of high-affinity IgE receptor I-mediated mast cell activation.

It enhances both early- and late-phase reactions. It is independent of the mechanism of high-affinity IgE receptor I activation.

In B cells, BTK is a critical regulator of B-cell receptor signaling as well as B-cell development and function.

Within the B-cell lineage, BTK is required for the progression of B-cell precursors beyond the pre-B cell stage.

BTK transcripts are detected in pre-B cells all the way to mature B cells.

And then the expression of the BTK gene is down-regulated at this transition from mature B cells to plasma cells.

Following engagement of the B-cell receptor, BTK is phosphorylated by the kinases Lyn and Syk, which promote its catalytic activity and subsequent autophosphorylation.

BTK catalytic activity in B cells drives the activation of several key signaling pathways, such as phospholipase C and NF kappa B, giving B cells a very strong survival signal upon B-cell receptor engagement.

BTK also facilitates antigen presentation, antibody production, and cytokine release from B cells.

Coming back to the role of BTK in CSU, BTK mediates mast cell as well as basophil signaling through the high-affinity IgE receptor, which is relevant to both type I as well as type IIb autoimmune CSU, and augments cellular degranulation, histamine release, cytokine production, and eicosanoid synthesis.

BTK is also relevant in the pathophysiology of autoimmune diseases in general, with increased BTK expression in B cells of patients with autoimmunity.

BTK acts in autoreactive B cells to promote proliferation and differentiation into pathogenic plasma cells, augmenting the production of autoreactive antibodies.

There is also preclinical evidence that autoreactive B cells are more exquisitely dependent on BTK for survival than normal B cells.

BTK is also involved in other pathways, including toll-like receptor – or TLR – signaling and antigen presentation to T cells, both of which can augment B cell-driven autoimmune processes.

To summarize, BTK enhances autoreactivity by increasing autoantibody production but also facilitating antigen presentation to T cells and increasing proinflammatory cytokine production.

Thus, BTK has a multifaceted role in the pathophysiology of chronic spontaneous urticaria, potentially through increased autoantibody production by autoreactive B cells, as well as activation of mast cells and basophils downstream of the high-affinity IgE receptor.

A common concern may be that inhibition of BTK may have the same clinical manifestation as X-linked agammaglobulinemia, or XLA.

It is important to note that targeting of BTK does not have the same effect on B cells as XLA.

BTK is not expressed in mature plasma cells or needed for their function and survival.

To summarize our presentation, autoallergic or autoimmune high-affinity IgE receptor I-dependent activation of mast cells and basophils plays a role in CSU pathophysiology.

BTK plays a major role in immune response and is a key mediator of B-cell receptor and high-affinity IgE receptor I-mediated signaling in mast cells and basophils.

Thank you for your attention.