Actin cytoskeleton-dependent down-regulation of early IgE-mediated signaling in human basophils

Investigational spleen tyrosine kinase (SYK) inhibitors for the treatment of autoimmune diseases

INTRODUCTION

Autoimmune diseases (ADs) are disorders induced by multiple inflammatory mediators, in which immune system attacks healthy tissues and triggers tissue injury. Targeted regulation of the activity of kinases that influence inflammation is one of the major therapies for ADs. Recently, investigational spleen tyrosine kinase (SYK) inhibitors have shown encouraging results in the ADs therapy.

AREAS COVERED

This article provides a background on autoimmune diseases and provides an update on investigational SYK inhibitors. This literature review was conducted by searching publications about investigational Spleen tyrosine kinase inhibitors in the treatment of ADs from experimental to clinical studies. The search terms used were SYK inhibitors, R406, fostamatinib (R788), P505-15 (PRT062607), entospletinib (GS-9973), R112, lanraplenib (GS-9876), cerdulatinib, R343, BAY-61-3606, GSK compound 143 (GSK143), R211, SKI-G-618, SKI-O-85, ER-27319, YM193306, RO9021 in conjunction with autoimmune disease using electronic databases including PubMed, EMBASE, MEDLINE and Google Scholar.

EXPERT OPINION

SYK inhibitors are promising drugs with unique advantages and acceptable tolerability and safety for the treatment of ADs. However, the difficulties in developing highly selective SYK inhibitors and the unknown effects are challenges. Long term and real-world data are essential to determine the risk-benefit ratio and true role of SYK inhibitors in the therapy of ADs.

Omalizumab increases the intrinsic sensitivity of human basophils to IgE-mediated stimulation

BACKGROUND

Treatment of allergic patients with omalizumab results in a paradoxical increase in their basophil histamine release (HR) response ex vivo to cross-linking anti-IgE antibody. It is not known whether this change in response is associated with an increase in intrinsic cellular sensitivity, which would be a paradoxical response.

OBJECTIVE

We sought to determine whether the increase in response to anti-IgE antibody is a reflection of an increased cellular sensitivity expressed as molecules of antigen-specific IgE per basophil required to produce 50% of the maximal response.

METHODS

Patients were treated with omalizumab or placebo for 12 weeks (NCT01003301 at ClinicalTrials.gov), and the metric of basophil sensitivity was assessed at 4 time points: baseline, 6 to 8 weeks, 12 weeks (after which treatment stopped), and 24 weeks (12 weeks after the end of treatment).

RESULTS

As observed previously, treatment with omalizumab resulted in a marked increase in the maximal HR induced by cross-linking anti-IgE antibody. This change was accompanied by a marked shift in intrinsic basophil sensitivity, ranging from 2.5- to 125-fold, with an average of 6-fold at the midpoint of the treatment to 12-fold after 12 weeks. The magnitude of the increase in cellular sensitivity was inversely related to the starting sensitivity or the starting maximum HR. The increased cellular sensitivity also occurred when using leukotriene C4 secretion as a metric of the basophil response. Twelve weeks after the end of treatment, cellular sensitivity was found to shift toward the baseline value, although the return to baseline was not yet complete at this time point.

CONCLUSIONS

Treatment with omalizumab results in a markedly increased sensitivity of basophils to IgE-mediated stimulation in terms of the number of IgE molecules required to produce a given response. These results provide a better quantitative sense of the phenotypic change that occurs in basophils during omalizumab treatment, which has both mechanistic and clinical implications.

Marked differences in the signaling requirements for expression of CD203c and CD11b versus CD63 expression and histamine release in human basophils

Many techniques are being used to examine the status of circulating human basophils including the enhanced expression of a variety of cell surface proteins. There is accumulating evidence that there are at least two compartments containing these activation marker proteins but there are only some indications for the signaling requirements for each of the compartments. This study began with published reports by other investigators who potentially dissociated CD63 expression from anaphylactic degranulation with the p38 inhibitor, SB203580, a possible falsification of a previously proposed hypothesis regarding CD63 expression. To explore the signaling requirements for CD63, a variety of pharmacological agents were used to inhibit or enhance 4 endpoints of basophil activation. First, it was found that inhibition of both histamine release and CD63 expression with SB203580 was concordant. But it was also found that this agent had no effect on increased expression of CD203c and CD11b. Actin polymerization inhibitors caused marked enhancement of CD63 expression (concordant with their effects on degranulation) with no effect on expression of CD203c and CD11b. The third generation syk inhibitor, NVP-QAB205, showed a 5-fold lower potency for inhibiting expression of CD203c and CD11b than for CD63. Finally, while desensitization of CD11b and CD203c expression occurs, it is slower than desensitization of the CD63 response. Taken together, these various observations demonstrate a marked difference in the early signaling requirements for the CD11b/CD203c compartment and CD63 degranulation and provide support for the hypothesis that CD11b and CD203c reside in a similar compartment.

Immunology in the Clinic Review Series; focus on allergies: basophils as biomarkers for assessing immune modulation

Allergen-specific immunotherapy is an effective clinical treatment for hypersensitivity to many allergens. Studies of basophils during immunotherapy have provided insight into underlying immune mechanisms and support the potential use of basophil activation as a biomarker of clinical outcomes. This review examines the evidence for different pathways of basophil modulation associated with various forms of immunotherapy. Better understanding the molecular mechanisms of basophil activation and desensitization and the relationship between suppression of these effector cells to clinical outcomes holds promise for further development and improvement in potential therapies for allergic diseases.

Early signal protein expression profiles in basophils: a population study

IgE-mediated histamine release from peripheral blood basophils is highly variable within the general population. Recent studies have shown that the ability of anti-IgE antibody to induce release can be predicted reasonably well by knowing the level of syk expression in the cells. The current study expands a previous survey to include 14 additional early elements known to be involved in activation and deactivation of basophils and showed that with the exception of syk, the variance of expression of 19 other elements (lyn, fyn, csk, cbp/PAG, CIN85, Bob1, c-cbl, SHIP1, SHIP2, p85alpha, p110delta, btk, PLCgamma1, PLCgamma2, SHP-1, PTEN, SOS2, CRACM1, and IL-3Ralpha) was narrow despite a broad range of functional capability in the basophils under study. With syk as the only element with high variance and well-correlated to maximum histamine release and cellular sensitivity, this survey examined the expression levels of two proteins thought to regulate syk expression: Bob1/OCA-B and CIN85. Expression of CIN85 was not correlated to syk expression, but Bob1 expression was negatively correlated to expression of syk and maximum histamine release. However, the expected behavior for this protein should have been as a protector of post-translational syk loss and therefore, positively correlated. Previous studies suggested that post-translational control mechanisms regulated syk expression. However, in this study, steady-state mRNA levels for syk in resting basophils showed a correlation with syk protein expression levels (r=0.593). It is concluded that with the exception of syk expression, the expression of 19 early signaling elements is tightly regulated and that a component of the regulation of syk may be related to control of transcription or processing of syk mRNA.

Polymerization of actin does not regulate desensitization in human basophils

Previous studies have suggested that maintenance of IgE-mediated signaling results from regulation of the activity of signaling complexes by actin polymerization. This process is also hypothesized to be related to desensitization of basophils and mast cells. Recent studies demonstrated that any signaling process dependent on syk or PI-3K activity cannot be a mechanism of desensitization, and in this context, syk and PI-3K inhibitors were found to inhibit actin polymerization. Inhibitors of actin polymerization were tested for their effect on desensitization of human peripheral blood basophils. Latrunculin A, in particular, removed all resting and stimulated f-actin but did not inhibit desensitization. Cytochalasin D and latrunculin A also did not reverse the loss of syk phosphorylation that accompanies desensitization. These results demonstrate that desensitization mechanisms are not dependent on actin polymerization. In this context, it was also shown that progressive immobilization of Fc epsilon RI during aggregation was sensitive to syk or actin polymerization inhibition. Therefore, desensitization is also not dependent on receptor immobilization. These studies demonstrate that desensitization is not the result of two signaling pathways once considered relevant to down-regulation of IgE-mediated signaling.

Relationship between spleen tyrosine kinase and phosphatidylinositol 5' phosphatase expression and secretion from human basophils in the general population

BACKGROUND

Previous studies have suggested that expression levels of spleen tyrosine kinase (syk) or phosphatidylinositol 5' phosphatase (SHIP) may explain certain extreme human basophil phenotypes.

OBJECTIVE

This study is designed to explore whether variability in syk and SHIP expression levels in the general population, alone or in concert, can account for the variability in basophil function.

METHODS

A survey of maximum responsiveness to IgE-mediated stimulation, sensitivity, and expression levels of 6 early signaling elements was performed on 36 subjects' basophils.

RESULTS

Of the 6 signaling elements, only syk and SHIP showed a correlation with maximum histamine release or cellular sensitivity. In a multiple regression, syk and SHIP together could account for 67% of population variance, although most of the variance was explained by syk expression. The pattern of expression variance syk>>SHIP1>SHIP2 approximately lyn approximately p85 approximately cbl suggested a process that primarily modulated syk levels. IL-3 is known to modulate syk levels, but we found that a 3-day incubation with IL-3 resulted in increased expression of other signaling elements to a greater degree: cbl>SHIP1>SHIP2 approximately lyn approximately p85 > or = syk, opposite the pattern in the population survey. In contrast, 18-hour stimulation with anti-IgE antibody led to marked downregulation of syk expression, modest downregulation of Fc epsilon RI expression, weak downregulation of lyn expression, and no effect on 23 other signaling elements.

CONCLUSION

Unlike studies in mice, we conclude that expression of syk is a good preconditioning predictor of basophil function in the general population.

CLINICAL IMPLICATIONS

The finding that expression of syk levels may strongly influence functional responses of basophils suggests a mechanism underlying the severity of atopic diseases.

Nonspecific desensitization, functional memory, and the characteristics of SHIP phosphorylation following IgE-mediated stimulation of human basophils

Previous studies of secretion from basophils have demonstrated the phenomenon called nonspecific desensitization, the ability of one IgE-mediated stimulus to alter the cell's response to other non-cross-reacting IgE-mediated stimuli, and a process that would modify phosphatidylinositol 3,4,5-phosphate levels was speculated to be responsible for nonspecific desensitization. The current studies examined the changes and characteristics of SHIP1 phosphorylation as a measure of SHIP1 participation in the reaction. Based on the earlier studies, two predictions were made that were not observed. First, the kinetics of SHIP1 phosphorylation were similar to reaction kinetics of other early signals and returned to resting levels while nonspecific desensitization remained. Second, in contrast to an expected exaggerated SHIP phosphorylation, cells in a state of nonspecific desensitization showed reduced SHIP phosphorylation (compared with cells not previously exposed to a non-cross-reacting Ag). Discordant with expectations concerning partial recovery from nonspecific desensitization, treatment of cells with DNP-lysine to dissociate bound DNP-HSA, either enhanced or had no effect on SHIP phosphorylation following a second Ag. These experiments also showed a form of desensitization that persisted despite dissociation of the desensitizing Ag. Recent studies and the results of these studies suggest that loss of early signaling components like syk kinase may account for some of the effects of nonspecific desensitization and result in a form of immunological memory of prior stimulation. Taken together, the various characteristics of SHIP phosphorylation were not consistent with expectations for a signaling element involved in nonspecific desensitization, but instead one which itself undergoes nonspecific desensitization.

Transient transfection of human peripheral blood basophils

The human basophil has resisted previous attempts at transient transfection. Basophils were transfected by nucleoporation and to test whether there was sufficient expression to modify cell function, the cells were transfected with a syk kinase tandem SH2 construct linked to GFP. This approach was taken because in RBL cells and murine mast cells syk kinase is known to play a very early role in signal transduction and previous studies in RBL cells demonstrated that expression of the tandem SH2 domains of syk would inhibit signaling, presumably by competition with endogenous syk for binding to ITAMs. Results from basophil transfections with SH2syk were compared to an empty construct. Basophils were stimulated with anti-IgE antibody and analyzed for single cell changes in cytosolic calcium levels. Basophils expressing the empty GFP construct showed a cytosolic calcium response similar to non-expressing cells. In contrast, basophils expressing the GFP-tandem SH2syk construct, on average, showed an anti-IgE-induced calcium response that was completely ablated. The transfection frequency was 8% (median), with an average viable recovery of 12% (n=18). While the procedure is not benign and is not always successful, these studies indicate that with gating techniques, the human basophil, a non-dividing primary leukocyte, can be transiently transfected to express high enough levels of an inhibitory protein to alter an IgE-mediated response.