Nonspecific Desensitization, Functional Memory, and the Characteristics of SHIP Phosphorylation following IgE-Mediated Stimulation of Human Basophils

Abstract

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.

Early signal protein expression profiles in basophils: a population study

“…The reactions in this signaling model have kinetics with enzymatic characteristics (a Vmax and Km). The distinction in steps is motivated by the knowledge that there is a step that precedes activation of syk, thought to be a src-family kinase member, which also appears to regulate the activation of SHIP, independent of syk activity [5]. syk is known to lead to activation of PI3K [19], and SHIP is known to regulate PIP3 levels [20], one possible output of the activity of PI3K.…”

“…The relationship between syk expression and histamine release is likely the result of syk being a critical, early, nonredundant signaling kinase in the signaling pathway and that syk is present in limiting amounts (the syk:FcRI ratio is 1:7). Various studies in human basophils support the criticality of syk in IgE-mediated signaling [5,6] and in a small study of syk substrate phosphorylation, the level of syk expression if basophils of different subjects were mirrored by the extent of phosphorylation of shc or c-cbl [4]. The pilot survey examined only six elements thought to be involved in IgE-mediated signaling, but the survey provided material for a broader study.…”

Induced Loss of Syk in Human Basophils by Non-IgE-Dependent Stimuli

“…8, C and D). For comparison, the IC 50 for anti-IgE-induced Cbl phosphorylation is shown and this IC 50 is the same as found for many other downstream targets of Syk activation (21).…”

“…Previous studies of the Novartis inhibitor NVP-QAB205 show that it displays the expected characteristics of Syk inhibitor with an IC 50 for inhibition of early signaling steps that lies somewhere between 0.01 and 0.1 M (typically, ϳ0.04 M) (21). Human basophils were stimulated with either anti-IgE Ab or anti-LILRA-2 Ab in the presence of NVP-QAB205 or equivalent concentrations of its DMSO carrier solvent.…”

“…Therefore, a related but more general hypothesis is that any receptor that generates an activated Cbl might also lead to loss of Syk protein. This possibility is more difficult to visualize because current models suggest that there must be interaction between Cbl and Syk for ubiquitination to occur and for the IgEmediated response in basophils, Cbl phosphorylation is a Syk-dependent event (21) and so is the autophosphorylation of Syk tyrosines (22). But, there is at least one example of a receptor that induces phosphorylation of c-Cbl without apparent phosphorylation of Syk.…”