John Thomas Schroeder, Ph.D.

This laboratory during the last two decades has focused on understanding the role human basophils and mast cells play in allergic reactions, as it relates not only to their secretion of potent inflammatory mediators (e.g. histamine and leukotriene C4), but also to their production of pro-inflammatory cytokines. Particular emphasis has been on studies investigating the parameters and mechanisms underlying the production of IL-4 and IL-13 by basophils, as these two cytokines are known to play a critical role in the pathogenesis of allergic disease. As a result, this laboratory has played a significant role in characterizing the basophil as a major source of these "Th2-like" cytokines, so named for their initial description in a subclass of T helper lymphocytes. We first demonstrated this response in vitro using basophils isolated from blood and under conditions involving IgE-dependent and IgE-independent activation. We have also extended these observations to clinically relevant disease by showing that basophils infiltrating the airways following allergen challenge are also a primary source of these cytokines. Overall, the implications are that basophils, by secreting all of the major products linked to allergic inflammation (i.e. IL-4, IL-13, histamine and LTC4), orchestrate both immunomodulatory and effecter roles in allergic disease. Consequently, this laboratory has also played a leading role in characterizing these responses in subjects suffering from diverse types of allergic disease, ranging from asthma to food allergy.

During the last decade this laboratory has also expanded its research interests to include dendritic cell (DC) biology, after observing unique phenotypic and functional similarities between basophils and a particular dendritic cell (DC) subtype referred to as the plasmacytoid DC (pDC). For example, pDCs express a variant of FcεRI (ag2), which enables these cells to bind IgE immunoglobulin with high affinity, much like basophils and mast cells. Importantly, our work has shown evidence for the existence of a unique counter-regulatory axis between FcεRI and a specific innate immune receptor (e.g. TLR9) co-expressed by pDC. In vitro studies show that activation of pDCs through FcεRI (as mediated by allergen) suppresses their capacity to respond to TLR9 agonists. In contrast, if pDC are first activated via TLR9 agonists, then they down-regulate FcεRI. Overall, this mechanism may very well explain why pDC from allergic subjects are impaired in responding to TLR9 agonists, which are known to activate substances [e.g. interferon (IFN)-alpha] that can suppress the Th2 responses linked to allergic inflammation. Likewise, the clinical efficacy seen in treating allergic disease with therapeutics targeting TLR9 (e.g. so-called CpG-based compounds) may result from their capacity to counter-regulate FcεRI responses in the pDC subtype.

Finally, this laboratory has long utilized human cells rather than cell lines in order to address the parameters, signal transduction, and pharmacological aspects underlying clinically relevant basophil, mast cell, and DC responses. As a result, this laboratory has established protocols for rapidly isolating relatively large numbers of these cell types at high purity from human blood, and for growing culture-derived mast cells from human progenitor cells. A variety of assays and techniques are also in place for concurrently detecting cytokines and mediators following a wide range of stimuli. These have thus facilitated the in vitro pharmacologic testing of numerous anti-allergic drugs for inhibitory activity on basophil, mast cell, and DC activation.