In the continuation of this grant, we propose further examination of the roles played by the collectins, SP-A and SP-D in naive and inflamed lungs. We earlier provided support for the concept that these molecules can act as dual-function discriminators to maintain a relatively anti-inflammatory environment in the resting lung, but also as innate immune system recognition molecules that can initiate inflammation and immune responses to injury and potential pathogens. These opposing effects were proposed to result from the suppressive actions of lung collectin head groups acting on the ITIM-containing receptor, SIRP on the one hand or, in the alternative orientation, pro-inflammatory effects from their collagenous tails interacting with a complex of calreticulin and LDL receptor related protein (LRP-1) on the other. In this renewal, the dual effects of the collectins and their candidate receptors will be pursued in the context of lung macrophages. We hypothesize that the resident alveolar macrophage phenotype is significantly driven by its exposure to the lung collectins, which maintain a tonic suppressive stimulation through SIRP and its downstream effectors, the SHP tyrosine phosphatases acting on inflammatory mediator production through, in part, inhibition of P38 MAPkinase. In this model, the sentinel function of these resident macrophages is represented by their ability to be unshackled from the suppression in response to injury and invasion. We propose that this unshackling of the suppressed state is caused in part by the reversed function of these same collectins now acting through their tails on LRP. Studies will be performed to address this balance using a variety of potential "danger" signals on resident alveolar macrophages (RAM) in vitro and in the naive mouse lung. These will be contrasted with "inflammatory" alveolar macrophages (IAM) that have emigrated into the lung during an inflammatory reaction and whose markedly different phenotype is proposed to reflect a reduced effect of the collectins and their stimulation of SIRP. We further suggest, and will explore, the persistence of resident alveolar macrophages throughout an inflammatory response with return to their sentinel function after resolution. These studies will involve a system in which the two types of macrophage in the mouse are distinguishable following irradiation (with shielding of the lungs) and transplant with GFP-tagged bone marrow. The inflammatory macrophages are suggested to undergo local apoptosis and removal during resolution of the inflammation, in part, because they are less effectively protected from apoptosis by the lung collectins, perhaps also acting through SIRP.
. The normal lung is constantly exposed to the external environment and needs to discriminate between relatively harmless exposure and potential dangerous insults. Resident alveolar macrophages are suggested to mediate an important element of this discrimination. We suggest that they are normally maintained in a quiet, somewhat suppressed state that can be released (unshackled) by danger signals to then initiate a protective inflammatory response. A potential mechanism for this is their response to the local lung collectins SP-A and SP-D. The objective of this proposal is to explore the fascinating dual functions of SP-A and SP-D, which in one orientation appear to suppress macrophage function and in another can recognize foreign insults and stimulate the macrophages to produce pro-inflammatory mediators. Accordingly we will examine their contribution to the unique phenotype of the resident alveolar macrophage with an ultimate objective of devising new interventional approaches to regulate pulmonary inflammatory reactions and their outcomes.
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