Introduction
Inflammatory cell recruitment is a key step in the initiation of the
acute immune response. Macrophage activation at the site of tissue
injury results in the secretion of chemokines; these have a
non-redundant role in leukocyte recruitment in pre-clinical models of
inflammation. Activated macrophages are a major source of
pro-inflammatory cytokines and chemokines in many chronic inflammatory
diseases including rheumatoid arthritis, diet-induced diabetes or
atherosclerosis (White et al., 2013). Chemokine-mediated recruitment,
retention and activation of leukocytes, are attractive areas for the
development of novel anti-inflammatory agents that could find
application in a wide range of chronic inflammatory pathologies. We and
others have demonstrated that genetic targeting of chemokine receptors
shows promise in pre-clinical models of acute and chronic inflammation
(McNeill et al., 2017). However, interventional studies in man using
chemokine receptor antagonists have so far had limited therapeutic
benefit (Schall and Proudfoot, 2011) (Noels et al., 2019). This could be
due to high redundancy of chemokine receptor use in leukocytes to
initiate monocyte/macrophage migration. Therefore, a therapeutic
strategy that would affect both a myeloid cell’s ability to undergo
chemotaxis and reduced chemokine secretion could limit leukocyte
recruitment by two independent but complementary mechanisms.
Bruton’s tyrosine kinase (BTK) is a non-receptor bound intracellular
signalling molecule best known for its signalling in B-cell development
and malignancy. However, in recent years an alternative role for BTK is
emerging in innate immune cells. BTK has been shown to be expressed at
relatively high levels in myeloid cells (Mangla et al., 2004). BTK is
activated in monocytes and macrophages in numerous acute inflammatory
conditions, including polymicrobial sepsis and cerebral ischaemia
(O’Riordan et al., 2019a)(Ito et al., 2015) but also in chronic
inflammatory conditions such as obesity-induced diabetes, rheumatoid
arthritis and lupus (Purvis et al., 2020)(Honigberg et al.,
2010)(Hartkamp et al., 2015). During the COVID-19 pandemic BTK has been
shown to be activated in monocytes and intervention with the BTK
inhibitor Acalabrutinib has been reported to reduce systemic
inflammation in patients with severe COVID-19 (Roschewski et al., 2020).
We and others have reported inhibition of BTK reduces NF-kB and the
NLRP3 inflammasome activity in both murine and human macrophages, and
reduces pro-inflammatory cytokine and chemokine production (O’Riordan et
al., 2020)(Mao et al., 2020).
Pharmacological inhibition of BTK reduced macrophage accumulation in
inflamed tissues in a pre-clinical model of type II diabetes (Purvis et
al. 2020) and a model of cerebral ischaemia (Ito et al 2015). To confirm
and extend these studies we tested the hypothesis that BTK can directly
regulate myeloid cell recruitment to sites of inflammation. To do this
we used a combination of in vivo and in vitro cell
recruitment assays using a range of European medicines agency (EMA) or
US Food and Drug administration (FDA) approved BTK inhibitors and tool
compounds in combination with BTK-deficient X-linked immunodeficient
(XID) mice. This allowed us to fully explore the magnitude and kinetics
of myeloid cell recruitment in acute inflammation. We demonstrate that
inhibition of BTK activity reduces neutrophil and monocyte recruitment
via two independent but complementary mechanisms a) reducing monocyte
chemotaxis, and b) reducing chemokine production from tissue resident
macrophages.