We have proposed previously that hemopoietic myeloid progenitors contribute to the ongoing recruitment of proinflammatory cells, namely eosinophils, to sites of allergen challenge in allergic diseases such as asthma. In this study, we investigated the involvement of bone marrow-derived progenitors in the development of allergen-induced pulmonary inflammation in mild asthmatic subjects. By flow cytometry, we enumerated the level of expression of CD34, a hemopoietic progenitor cell marker, on bone marrow aspirates taken before and 24 h after allergen challenge. In addition, the coexpression of the ␣ -subunits of IL-3 receptor (IL-3R) and IL-5 receptor (IL-5R) on CD34 ϩ cells was investigated. After allergen-challenge, although no significant change in total BM CD34 ϩ cell numbers was observed, a significant increase in the proportion of CD34 ϩ cells expressing IL-5R ␣ , but not IL-3R ␣ , was detected in the 24-h post-allergen, compared with the pre-allergen bone marrow. This was associated with a significant blood and sputum eosinophilia and increased methacholine airway responsiveness, 24 h post-allergen. Using simultaneous in situ hybridization and immunocytochemistry, we colocalized the expression of messenger RNA for membrane-bound IL-5R ␣ to CD34 ϩ cells. In summary, our data suggest that increased expression of IL-5R ␣ on CD34 ϩ cells favors eosinophilopoiesis and may thus contribute to the subsequent development of blood and tissue eosinophilia, a hallmark of allergic inflammation. ( J. Clin. Invest. 1997. 100:2466-2475.)

Increases in inflammatory cell progenitors, particularly eosinophil/basophil colony-forming cells (Eo/B-CFU), occur in peripheral blood after allergen provocation. The role of bone marrow (BM) in these reactions is unclear. We examined the effect of allergen challenge on human bone marrow progenitor cell growth. Fifteen asthmatic subjects, eight dual responders (DR) and seven isolated early responders (IER), were challenged with inhaled allergen. BM aspirates were taken before and 24 h after challenge and progenitors were enumerated by a colony-forming assay. Eo/B-CFU numbers increased in both groups after allergen challenge (p < 0.0001). For DR, the increases were significant for BM incubated with optimal GMCSF and IL-5, but not with IL-3. For IER, the increases were significant for all three cytokines tested. At a suboptimal concentration of IL-5, there was a significant increase in the number of Eo/B-CFU after allergen in the DR, from 5.25 +/- 1.2 to 9.68 +/- 2.1 per 2.5 x 10(5) cells plated (p < 0.01), which was not demonstrated in the IER (p = 0.94). The responses at this concentration of IL-5 were different between groups (p < 0.05). These results demonstrate that inhaled allergen increases BM Eo/B-CFU, and that the bone marrow of dual responders is more responsive to IL-5 after allergen.

Inhaled allergen challenge of subjects with atopic asthmatic increases bone marrow eosinophil progenitor cells. Interleukin-5 (IL-5) specifically induces growth and maturation of eosinophils. This study examined the effect of allergen challenge on the number of bone marrow total and CD3+ cells expressing IL-5 protein and IL-5 mRNA in subjects with asthma who developed either allergen-induced isolated early responses, or early and late asthmatic responses (dual responders). At 24 hours after allergen challenge, dual responders had significantly greater blood and airway eosinophilia compared with early responders. There were significant increases in the percentage of bone marrow CD3+ cells (p < 0.005) in both groups. However, there were significant differences in the increases in bone marrow IL-5 mRNA+ (p < 0.005), CD3+ (p < 0.005), and IL-5 mRNA+ CD3+ (p < 0.005) cells between the dual and early responder groups. These results suggest that, in subjects with atopic asthma, inhaled allergen causes trafficking of T lymphocytes to the bone marrow, and that in subjects who develop late responses and greater blood and airway eosinophilia after inhalation of allergen, there is a significant increase in the ability of bone marrow cells, particularly T lymphocytes, to produce IL-5.