* 0.05; ** 0.01; n.s., not significant. Next, we tested the uptake of the bacteria opsonized by nIgG:ficolin immune complexes, by monocytes. proteins. These results provide mechanistic insights on the interaction between two molecules representing the adaptive and innate immune pathways, prompting potential development of immunomodulatory/prophylactic peptides tunable to prevailing infection conditions. In order to combat pathogens, the CHMFL-ABL-039 host has evolved an elaborate immune system comprising of two arms: innate and adaptive1, which are conventionally known to act in a biphasic manner. Although appearing separate and sequential, the interaction between proteins of the innate and adaptive immune pathways has been shown to shape the adaptive immune response2. For example, mannose binding lectin (MBL, a soluble innate immune PPR) binds to adaptive immune molecules such as antigen-specific IgG in immune complexes3, antigen-specific IgM4 and secretory IgA5, to facilitate the clearance of the CHMFL-ABL-039 opsonized microbes through activation of the complement pathway and prime the subsequent adaptive response. However, little is known about the molecular mechanisms by which adaptive immunity may fine-tune the innate immune responses, since the latter is deemed to be the frontline defense. The potential interactions between the proteins of the adaptive and innate arms of immunity remain an important area to be explored. In contrast to the well-known antigen-specific antibodies that are produced specifically during the adaptive immune response to an infection, there is a pool of non-specific naturally occurring antibodies comprising of IgM, IgG and IgA subtypes, which exists prior to an external infection. Amongst the natural antibody isotypes, the natural IgM has been most well-studied. The natural IgM was shown to possess non-specific avidity for pathogens, by virtue of its pentameric structure6,7, which enables it to exhibit a protective effect during infections8,9. Natural IgG (henceforth referred to as nIgG) belongs to the IgG3 subclass. Although nIgG makes up the majority of the serum natural antibodies10,11, the significance of its existence and function remained unexplored. Recently, Panda et al12 showed that nIgG (deemed to be an adaptive immune protein) collaborates with major serum lectins like ficolin and MBL, to immediately elicit host defense. It was demonstrated that nIgG specifically collaborates with ficolin (a pattern recognition receptor belonging to the lectin family of soluble PRRs) that is pre-bound to the pathogen, resulting in effective recognition and opsonization of the invading pathogen. The resulting nIgG:ficolin immune complex bound on the pathogen evokes innate immune defense, clearing the pathogen through FcR1-mediated phagocytosis. The H-ficolin was shown to be the most effective of the ficolin isoforms. Further studies demonstrated the protective role of nIgG. Mice lacking nIgG showed significantly higher bacterial burdens in the tissues, delay in bacterial clearance, increased pro-inflammatory and lowered anti-inflammatory cytokine production and compromised survival post-infection. Reconstitution with nIgG restored nIgG:ficolin mediated bacterial recognition and clearance and improved survival12. These findings prompted us to probe the dynamics of nIgG:ficolin interaction during an infection. In this study, we explored the interaction between the nIgG and H-ficolin under simulated physiological (pH 7.4, 2.5?mM calcium) and infection-inflammation (pH 6.5, 2.0?mM calcium) conditions. We showed that the Fc domain of nIgG specifically interacts with the FBG domain of ficolin. We further delineated the specific binding interfaces and peptides involved in interaction under normal and infection-inflammation conditions. Specific arginine and lysine residues were identified to be responsible for regulating basal interaction under normal condition, whereas histidine appeared to be crucial in increasing the affinity of nIgG:ficolin interaction under the infection-inflammation condition. Our results reveal novel insights into how adaptive immunity shapes innate immunity through molecular crosstalk between the proteins of the two arms of immunity. Identification of the cognate interactive peptides prompts future development of immunomodulators, which are tunable by pH CHMFL-ABL-039 and calcium changes in the microenvironment of infection. Results Natural IgG complexes with ficolin to recognize bacteria during infection Recently, we found that nIgG (belonging to the IgG3 subclass), present in the uninfected serum and deemed to be inactive10,11, actually CHMFL-ABL-039 plays an important Rabbit Polyclonal to Glucagon part in the immediate.