Cellular immune response parameters that influence IgE sensitization

Activation of Human Basophils by A549 Lung Epithelial Cells Reveals a Novel IgE-Dependent Response Independent of Allergen

Evidence for epithelial cell (EC)-derived cytokines (e.g., thymic stromal lymphopoietin [TSLP]) activating human basophils remains controversial. We therefore hypothesize that ECs can directly activate basophils via cell-to-cell interaction. Basophils in medium alone or with IL-3 ± anti-IgE were coincubated with TSLP, IL-33, or IL-25. Analogous experiments cocultured basophils (1-72 h) directly with EC lines. Supernatants were tested for mediators and cytokines. Abs targeting receptors were tested for neutralizing effects. Lactic acid (pH 3.9) treatment combined with passive sensitization tested the role of IgE. Overall, IL-33 augmented IL-13 secretion from basophils cotreated with IL-3, with minimal effects on histamine and IL-4. Conversely, basophils (but not mast cells) released histamine and marked levels of IL-4/IL-13 (10-fold) when cocultured with A549 EC and IL-3, without exogenous allergen or IgE cross-linking stimuli. The inability to detect IL-33 or TSLP, or to neutralize their activity, suggested a unique mode of basophil activation by A549 EC. Half-maximal rates for histamine (4 h) and IL-4 (5 h) secretion were slower than observed with standard IgE-dependent activation. Ig stripping combined with passive sensitization ± omalizumab showed a dependency for basophil-bound IgE, substantiated by a requirement for cell-to-cell contact, aggregation, and FcεRI-dependent signaling. A yet unidentified IgE-binding lectin associated with A549 EC is implicated after discovering that LacNAc suppressed basophil activation in cocultures. These findings point to a lectin-dependent activation of basophil requiring IgE but independent of allergen or secreted cytokine. Pending further investigation, we predict this unique mode of activation is linked to inflammatory conditions whereby IgE-dependent activation of basophils occurs despite the absence of any known allergen.

Isolation of Human Basophils

Isolating human basophils from blood has long been hampered by the fact that these granulocytes represent just 1% or less of the circulating leukocyte population. We describe herein laboratory protocols that have been refined over the past ∼25 years that now enable investigators to prepare basophils for use in a variety of assays to assess the in vitro biology of these immune cells, both in IgE -dependent and -independent responses.

IL-3 synergises with basophil-derived IL-4 and IL-13 to promote the alternative activation of human monocytes

Basophil-derived IL-4 is involved in the alternative activation of mouse monocytes, as recently shown in vivo. Whether this applies to human basophils and monocytes has not been established yet. Here, we sought to characterise the interaction between basophils and monocytes and identify the molecular determinants. A basophil-monocyte co-culture model revealed that IL-3 and basophil-derived IL-4 and IL-13 induced monocyte production of CCL17, a marker of alternative activation. Critically, IL-3 and IL-4 acted directly on monocytes to induce CCL17 production through histone H3 acetylation, but did not increase the recruitment of STAT5 or STAT6. Although freshly isolated monocytes did not express the IL-3 receptor α chain (CD123), and did not respond to IL-3 (as assessed by STAT5 phosphorylation), the overnight incubation with IL-4 (especially if associated with IL-3) upregulated CD123 expression. IL-3-activated JAK2-STAT5 pathway inhibitors reduced the CCL17 production in response to IL-3 and IL-4, but not to IL-4 alone. Interestingly, monocytes isolated from allergen-sensitised asthmatic patients exhibited a higher expression of CD123. Taken together, our data show that the JAK2-STAT5 pathway modulates both basophil and monocyte effector responses. The coordinated activation of STAT5 and STAT6 may have a major impact on monocyte alternative activation in vitro and in vivo.