The Role of Surfactant Protein A in Eosinophil Apoptosis and Clearance

Abstract

Surfactant protein A (SP-A) is the most abundant of the four surfactant proteins associated with pulmonary surfactant. Humans have two functional SP-A genes, SP-A1 and SP-A2, that organize to form an octadecamer, very similar in structure to other collectins, such as mannose-binding lectin. Although produced mainly in the distal airway by type II alveolar cells, it is synthesized independently of pulmonary surfactant in the conducting airways by club cells and cells in the submucosal glands. Like the other members of the collectin superfamily, SP-A is an important innate immune mediator. In fact, several studies have shown that SP-A plays a role in the regulation of host immune responses to type 2-associated allergen-induced inflammation. Eosinophils are well-known end-stage effector cells highly associated with type 2 high asthma, contributing to mucus production, bronchoconstriction and overall airway inflammation. The large success of inhaled corticosteroid (ICS) therapy, aiding in reducing eosinophil viability through the inhibition of eosinophil-specific chemokine and cytokine production, underline the importance of eosinophil clearance in the resolution of type 2-associated airway inflammation. Despite ICS therapy being a mainstay treatment for asthma, refractory asthma remains difficult to manage. Consequently, identifying alternative treatment approaches for these subgroups of asthmatics continue to be relevant. In these studies, we show that SP-A inhibits eosinophil degranulation in the context of Mycoplasma pneumoniae (Mp) infection, as well as, promotes eosinophil apoptosis in the context of allergic airway inflammation. These capabilities are altered by a specific genetic variation that occurs in the carbohydrate recognition domain (CRD) of SP-A, where a glutamine (Q) is substituted for a lysine (K) on SP-A2 at position 223 (SP-A2 Q223K). Moreover, we show that administration of exogenous SP-A rescues the persistent eosinophilia observed during experimental allergen challenge, promoting the apoptosis of eosinophils. Building from this work, we used quantitative proteomics to identify a novel membrane-associated interacting partner for SP-A on the eosinophil, myeloid-associated differentiation marker (MYADM). We likewise provide evidence for a functional role for the putative SP-A:MYADM interaction in eosinophil apoptosis and airway hyperresponsiveness in the setting of a type 2 inflammation. In vitro blockade of MYADM results in the abrogation of the previously observed eosinophil cell death induced by SP-A. Additionally, HDM-challenged wild-type mice that received the MYADM blockade exhibited delayed resolution of airway resistance. The loss of activity by SP-A in eosinophil degranulation and apoptosis due to the SPA-2 Q223K genetic variation in the CRD led us to hypothesize that the CRD is an important region of activity. To test the feasibility of using small molecule derivatives of from this active site, peptides and a series of peptidomimetics derived from the CRD of SP-A were developed and screened for their cytotoxic effect on eosinophils. Here, we also report the identification of four lead peptidomimetics that induce a robust decline in eosinophil viability in vitro, which will be further optimized for future testing in pre-clinical mouse models of asthma. Thus, results from this work demonstrate the role of SP-A in eosinophil degranulation, apoptosis and resolution of airway inflammation which is altered due to the specific SP-A2 Q223K genetic variation. These activities are potentially mediated through the newly identified SP-A:MYADM axis, where disruption of this axis results in prolonged eosinophilia and airway hyperresponsiveness. We also provide proof-of-concept for the potential of small molecule derivatives of SP-A to be used as a new class of therapeutics to target eosinophilic-driven inflammation such as in type 2 high asthma.