PDE4A and allergic asthma: In this regard, however, it is noteworthy that, notwithstanding the limited number of asthmatic cell lines examined, our results concur with those in previous studies, including the aforementioned study that also reported significantly increased intrinsic PDE4 activity in cultured HASM cells isolated from asthmatic individuals relative to normal non-asthmatic HASM cells [27], as well as the studies that implicated upregulated PDE4 actvity in mediating the in vivo airway responses to allergen challenge in asthmatic individuals [19,21] and in animal models of allergic asthma [18,22–26].