Individual IL-3 priming is crucial for consistent in vitro activation of donor basophils in patients with chronic urticaria.

BACKGROUND: The in vivo autologous serum skin test (ASST) is the diagnostic gold standard to detect autoantibodies against FcεRI or IgE itself, as well as other autoreactive serum components, in patients with chronic spontaneous urticaria (CU). Coincubation of patient sera with donor basophils and measuring their degranulation in vitro could be a safe alternative but has shown inconsistent results. OBJECTIVE: Optimization of the basophil activation test to detect autoreactive serum components in patients with CU. METHODS: The ability of patient sera to induce CD63 and CD203c in donor basophils (n = 15) was measured by means of flow cytometry. Sera of 20 patients with CU (10 with positive ASST results), 15 patients with cold urticaria, and 27 healthy control subjects were included to optimize test conditions with donor basophils and a basophil cell line (RBL703/21) followed by testing of 110 consecutive patients from clinical routine. RESULTS: We demonstrate that individual IL-3 priming normalized the initially inconsistent basophil reactivity and led to reproducible and comparable test results irrespective of the basophil donors used. CD203c as an activation marker and the use of a basophil cell line were less suitable for this purpose. CONCLUSION: The basophil activation test with individualized IL-3 priming for each basophil donor is a reproducible and reliable alternative to the ASST. There are several advantages over the ASST: no risk of accidental infection, no influence of antihistamines on the test result, quantifiable results, and a potential in providing treatment monitoring. The exact nature of the degranulating factor or factors in patient sera remains an open question.

Human basophils and eosinophils are the direct target leukocytes of the novel IL-1 family member IL-33.

In mice, interleukin-18 (IL-18) regulates Th1- or Th2-type immune responses depending on the cytokine environment and effector cells involved, and the ST2-ligand, IL-33, primarily promotes an allergic phenotype. Human basophils, major players in allergic inflammation, constitutively express IL-18 receptors, while ST2 surface expression is inducible by IL-3. Unexpectedly, freshly isolated basophils are strongly activated by IL-33, but, in contrast to mouse basophils, do not respond to IL-18. IL-33 promotes IL-4, IL-13 and IL-8 secretion in synergy with IL-3 and/or FcepsilonRI-activation, and enhances FcepsilonRI-induced mediator release. These effects are similar to that of IL-3, but the signaling pathways engaged are distinct because IL-33 strongly activates NF-kappaB and shows a preference for p38 MAP-kinase, while IL-3 acts through Jak/Stat and preferentially activates ERK. Eosinophils are the only other leukocyte-type directly activated by IL-33, as evidenced by screening of p38-activation in peripheral blood cells. Only upon CD3/CD28-ligation, IL-33 weakly enhances Th2 cytokine expression by in vivo polarized Th2 cells. This study on primary human cells demonstrates that basophils and eosinophils are the only direct target leukocytes for IL-33, suggesting that IL-33 promotes allergic inflammation and Th2 polarization mainly by the selective activation of these specialized cells of the innate immune system.