Identification of common and distinct origins of human serum and breastmilk IgA1 by mass spectrometry-based clonal profiling

The most abundant immunoglobulin present in the human body is IgA. It has the highest concentrations at the mucosal lining and in biofluids such as milk and is the second most abundant class of antibodies in serum. We assessed the structural diversity and clonal repertoire of IgA1-containing molecular assemblies longitudinally in human serum and milk from three donors using a mass spectrometry-based approach. IgA-containing molecules purified from serum or milk were assessed by the release and subsequent analysis of their Fab fragments. Our data revealed that serum IgA1 consists of two distinct structural populations, namely monomeric IgA1 (∼80%) and dimeric joining (J-) chain coupled IgA1 (∼20%). Also, we confirmed that IgA1 in milk is present solely as secretory (S)IgA, consisting of two (∼50%), three (∼33%) or four (∼17%) IgA1 molecules assembled with a J-chain and secretory component (SC). Interestingly, the serum and milk IgA1-Fab repertoires were distinct between monomeric, and J-chain coupled dimeric IgA1. The serum dimeric J-chain coupled IgA1 repertoire contained several abundant clones also observed in the milk IgA1 repertoire. The latter repertoire had little to no overlap with the serum monomeric IgA1 repertoire. This suggests that human IgA1s have (at least) two distinct origins;one of these produces dimeric J-chain coupled IgA1 molecules, shared in human serum and milk, and another produces monomeric IgA1 ending up exclusively in serum.

TREM-1 multimerization is essential for its activation on monocytes and neutrophils

The triggering receptor expressed on myeloid cells-1(TREM-1)is a receptor expressed on innate immune cells.By promoting the amplification of inflammatory signals that are initially triggered by Toll-like receptors(TLRs),TREM-1 has been characterized as a major player in the pathophysiology of acute and chronic inflammatory diseases,such as septic shock,myocardial infarction,atherosclerosis,and inflammatory bowel diseases.However,the molecular events leading to the activation of TREM-1 in innate immune cells remain unknown.Here,we show that TREM-1 is activated by multimerization and that the levels of intracellular Ca 2+release,reactive oxygen species,and cytokine production correlate with the degree of TREM-1 aggregation.TREM-1 activation on primary human monocytes by LPS required a two-step process consisting of upregulation followed by clustering of TREM-1 at the cell surface,in contrast to primary human neutrophils,where LPS induced a rapid cell membrane reorganization of TREM-1,which confirmed that TREM-1 is regulated differently in primary human neutrophils and monocytes.In addition,we show that the ectodomain of TREM-1 is able to homooligomerize in a concentration-dependent manner,which suggests that the clustering of TREM-1 on the membrane promotes its oligomerization.We further show that the adapter protein DAP12 stabilizes TREM-1 surface expression and multimerization.TREM-1 multimerization at the cell surface is also mediated by its endogenous ligand,a conclusion supported by the ability of the TREM-1 inhibitor LR12 to limit TREM-1 multimerization.These results provide evidence for ligand-induced,receptor-mediated dimerization of TREM-1.Collectively,our findings uncover the mechanisms necessary for TREM-1 activation in monocytes and neutrophils.