Targeting chemokine receptors signalling of immune cells in autoimmune diseases

The study of molecular signalling pathways in the fate of immune cells has gained relevance for their potential dysregulation in the onset of diseases of the immune system. Indeed, the signalling and metabolic pathways involved in the proliferation, development, and differentiation of different immune cell types have attracted the interest of the scientific and patient communities. In recent years, new evidence about the involvement of particular cell populations in the development of autoimmune disorders has been shown. An example is given by T cells, which metabolic status we have recently proposed to be the tipping point underlying the imbalance among T cell types in healthy versus immunocompromised patients.1 Thus, understanding the context of the development of T and B cell subtypes in different tissues, and the cues responsible for their emergence, appears to be crucial when treating autoimmune disorders. Recently, the work of Liu and colleagues presented new data that support the role of the Mst1/Akt/STAT axis in the development of central and peripheral B cell populations in CC chemokine receptor type 2 (CCR2) wild-type and knockout mice.2 CCR2 is a chemokine receptor that transduces signals in inflammatory diseases and the inflammatory response against cancer, upon binding of the chemokine (C-C motif) ligand 2 (CCL2) that mediates monocyte chemotaxis. Liu and colleagues observe an increased CCR2 expression in the peripheral blood mononuclear cells (PBMC) of systemic lupus erythematosus (SLE) patients compared to healthy individuals.2 This study follows the line of another paper published earlier by the same group about the role of the CCL2 ligand in modulating the B cell receptor (BCR) signalling and B cell differentiation.3 The new work provides a large amount of data supporting the function of CCR2 in the development of B cell populations, mainly impacting the development of peripheral populations, and decreasing the circulation of plasma cells and memory cells. Furthermore, a characterization of the pathways triggered by CCR2 stimulation shows evidence that mTORC1 is more active in CCR2 knockout mice B cells. Also, and related to the results shown in their early work,3 the authors observe a clear actin remodelling in the cells upon BCR stimulation that appears to be related to Mst1 activation. Taking all the evidence together, CCR2 stimulation may be considered a checkpoint for B cell activation and proliferation. In absence of CCR2, B cells are more prone to be stimulated and acquire a proliferative phenotype, accompanied by mTORC1 activation. Subsequently, stimulation of glucose metabolism occurs via HIF-1α activation, an increase of GLUT1 expression, as well as the flux through the tricarboxylic acid cycle, and oxidative phosphorylation is observed in activated phenotypes.4, 5 These B cells that are more prone to be activated exhibit a more mobile phenotype (due to an increase in the number and length of filopodia – thin membrane protrusions that are used by cells to probe their environment – as observed in CCR2 knockout mice B cells), yet with more difficulties to migrate to peripheral tissues and maturate. It is known that filipodia may be used by pathogens to evade the host immune system, being transported between the host cell filopodia induced by the pathogen during the initial stage of infection. Therefore, these particularly active (CCR2 knockout) cells might be more prone to respond to the nearest surrounding environment, thus having a higher chance to become reactive against host cells and lead to immune disorders. Altogether, cell populations of more dedifferentiated phenotypes are observed, and more mature circulating subtypes are reduced. These Ccr2-/- cells show higher levels of mTOR and Akt activation, as well as an overall higher level of phosphorylated targets, and appear to be involved in the emergence of autoimmune disorders.2 This outcome correlates with a low expression of CCR2 in PBMC observed in SLE patients compared to healthy controls. Although the specific role of CCR2 in SLE development has still to be elucidated yet, this work shows evidence that the decreased expression of CCR2 is correlated with the emergence of autoreactive cells that escape the peripheral tolerance mechanism. These cells are more prone to abnormal proliferation, potentially being ultimately responsible for the appearance of autoimmune disorders, such as SLE itself. Consistently, there is evidence that the level of mTOR activation in SLE patients correlates with the activity of the disease.6 Interestingly, other chemokine receptors are involved in the regulation of metabolic reprogramming and actin remodelling upon BCR stimulation, such as the CX3C chemokine receptor 1(CX3CR1). CX3CR1 binds the CX3CL1 chemokine ligand (known as neurotactin or fractalkine) which is involved in the adhesion and migration of lymphocytes. Similar to the observations about CCR2, the knockout of CX3CR1 in mice B cells leads to the overactivation of mTORC1 upon BCR stimulation.7 This outcome may seem to contradict the functioning of both chemokine receptors, as their action is also mediated by activation of the mTOR/Akt axis. However, the absence of the receptors enhances, instead of impairing, the activation of the signalling pathway and the consequent metabolic reprogramming. This evidence highlights the importance not only of the presence or absence of specific ligand-receptor interactions on the cell surface but also of the quantitative strength of this interaction which, along with the internal state of the cell, determines the quality of the immune response. For example, depending on the cellular context, mTOR activation may serve as a checkpoint before triggering the metabolic changes to sustain growth. However, a higher level of activity can lead to uncontrolled proliferation and activation of cells, resulting in the emergence of cancer or autoimmune disorders. A similar trend can be observed with other signalling pathways involved in the immune response, for example, the PI3K cascade. This pathway is involved in signal transduction upon BCR stimulation, recruiting Akt to the plasma membrane and, in turn, bridging the membrane events with the intracellular environment.8 This signalling is necessary for both the survival of naïve B cells and the development of humoral response, but some of its functions may seem contradictory. For example, PI3K activation is needed in early B cell development, as it activates IL7RA and Rag expression in pro-B cells (B cell fraction with non-rearranged BCR). However, PI3K suppresses Rag expression, via Akt-dependent FoxO1 inactivation to allow progression to the pre-B cell stage (when heavy and surrogate light chains of the BCR are expressed on the cell surface, yet with a non-complete rearranged BCR). The mechanism is the occurrence of dual phosphorylation of Akt through mTORC2, which allows for the timely transition from pro-B cell to pre-B cell. Figure 1 shows the signalling pathways involved in the B cell response upon membrane receptor activation. The work of Liu and colleagues can be seen as a continuation of a wider research area that studies the role of chemokine receptors in immune cell phenotypes and signalling, to understand better how autoimmune disorders may develop. Early studies have shown that peripheral blood CD4+ T cells (Th1) expressing CX3CR1 were increased in patients with rheumatoid arthritis (RA).9 More recently, peripheral blood and cerebrospinal fluid in B cells – naïve B cells in the former, antibody-secreting cells in the latter – expressed high levels of CX3CR3, another chemokine receptor that primarily regulates Th1 cell maturation upon ligand binding, in patients with neuromyelitis optica spectrum disorder (NMOSD), a devastating inflammatory disease of the central nervous system (CNS).10 Furthermore, independent studies found that B cells expressing CXCR3 are abundant in the CNS of multiple sclerosis (MS) patients,11 similarly to CXCR3 expressing T cells.12 In addition to the potential relevance of the chemokine receptor-mediated signalling, we want to point out the relevance of the immune metabolism as a modulator and target on its own, rather than the consequence of the activation of signalling pathways.13 There is evidence about the link between B cells and the metabolic programs to sustain a particular B cell phenotype or to switch from a B cell phenotype to another. For example, inhibition of glycolysis in B cells results in low proliferation and survival rates. In addition, activated cells are more sensitive to the limitation of glucose uptake than naïve B cells, which rely also on fatty acid oxidation to meet their metabolic demands.4 Therefore, the idea of reshaping the proportions of B cell subpopulations as a strategy to target disease development – as we have recently proposed for T cell phenotypes1 – could be investigated. This way, it may be possible to target different disease-associated subtypes based on their metabolic hallmarks.8, 13 Caution should be taken in the interpretation of results presented by Liu and colleagues because most of the experiments were performed in vitro using cells recovered from spleens, bone marrow, and the peritoneal cavity of mice.2 The CCR2 levels were evaluated in both SLE patients and healthy individuals; however, further characterization is needed to prove that the Mst1/Akt/STAT axis works the way suggested by the results in mice. It is in fact crucial to demonstrate that the proposed mechanism would work in healthy conditions and would be altered in SLE patients, as well as to show which implications it may have in the development and pathogenesis of the disease. Nevertheless, these results are promising to shed light on the chemokine receptor signalling as a new therapeutic target to ameliorate SLE symptoms, as well as those of other chronic autoimmune disorders. This work was supported by the Systems Biology Grant of the University of Surrey to Matteo Barberis. Alejandra Rojas López was supported by the Systems Biology of Cell Cycle Control studentship of the Faculty of Health and Medical Sciences (FHMS) of the University of Surrey to Matteo Barberis. The authors declare no conflict of interest.