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The complement system is central to first-line defense against invading pathogens. However, excessive complement activation and/or the loss of complement regulation contributes to the development of autoimmune diseases, systemic inflammation, and thrombosis. One of the three pathways of the complement system, the alternative complement pathway, plays a vital role in amplifying complement activation and pathway signaling. Complement factor D, a serine protease of this pathway that is required for the formation of C3 convertase, is the rate-limiting enzyme. In this review, we discuss the function of factor D within the alternative pathway and its implication in both healthy physiology and disease. Because the alternative pathway has a role in many diseases that are characterized by excessive or poorly mediated complement activation, this pathway is an enticing target for effective therapeutic intervention. Nonetheless, although the underlying disease mechanisms of many of these complement-driven diseases are quite well understood, some of the diseases have limited treatment options or no approved treatments at all. Therefore, in this review we explore factor D as a strategic target for advancing therapeutic control of pathological complement activation.

Type 2 diabetes is characterized by insulin resistance and a gradual loss of pancreatic beta cell mass and function 1 , 2 . Currently, there are no therapies proven to prevent beta cell loss and some, namely insulin secretagogues, have been linked to accelerated beta cell failure, thereby limiting their use in type 2 diabetes 3 , 4 . The adipokine adipsin/complement factor D controls the alternative complement pathway and generation of complement component C3a, which acts to augment beta cell insulin secretion 5 . In contrast to other insulin secretagogues, we show that chronic replenishment of adipsin in diabetic db / db mice ameliorates hyperglycemia and increases insulin levels while preserving beta cells by blocking dedifferentiation and death. Mechanistically, we find that adipsin/C3a decreases the phosphatase Dusp26 ; forced expression of Dusp26 in beta cells decreases expression of core beta cell identity genes and sensitizes to cell death. In contrast, pharmacological inhibition of DUSP26 improves hyperglycemia in diabetic mice and protects human islet cells from cell death. Pertaining to human health, we show that higher concentrations of circulating adipsin are associated with a significantly lower risk of developing future diabetes among middle-aged adults after adjusting for body mass index (BMI). Collectively, these data suggest that adipsin/C3a and DUSP26-directed therapies may represent a novel approach to achieve beta cell health to treat and prevent type 2 diabetes. Targeting the adipokine adipsin and its downstream pathway may provide an approach for preservation of beta cell loss in type 2 diabetes.

Tumor microenvironment plays a key role for tumor development and progression. Although adipose tissue is a predominant component of stroma in mammary tissues and secretes various cytokines, chemokines and growth factors, roles of adipocytes in breast cancers remain to be elucidated. In this study, we found that adipsin, an adipokine secreted from mammary adipose tissues, enhanced proliferation and cancer stem cell (CSC)-like properties of human breast cancer patient-derived xenograft (PDX) cells. Adipsin was predominantly expressed in both adipose tissues of the surgical specimens of breast cancer patients and adipose-derived stem cells (ADSCs) isolated from them, and its expression level was significantly higher in obese patients. ADSCs significantly enhanced the sphere-forming ability of breast cancer PDX cells derived from both estrogen receptor-positive and -negative breast cancer PDX cells. Suppression of adipsin-mediated signaling by a specific inhibitor or adipsin knockdown in ADSCs significantly decreased the sphere-forming ability and the expression of CSC markers in co-cultured breast cancer PDX cells. Growth of breast cancer PDX tumors was significantly enhanced by co-transplantation with ADSCs in vivo, and it was weakened when co-transplanted with the adipsin knocked-down ADSCs. These results suggest that adipsin is an important adipokine secreted from mammary adipose tissue that functions as a component of tumor microenvironment and a CSC niche in breast cancers.

Excessive intake of fat causes accumulation of fat in liver, leading to non-alcoholic fatty liver disease (NAFLD). High-fat diet (HFD) upregulates the expression of Factor D, a complement pathway component, in the liver of mice. However, the functions of Factor D in liver are not well known. Therefore, the current study investigated the relationship between Factor D and hepatic lipid accumulation using CRISPR/Cas9-mediated Factor D knockout (FD-KO) mice. Factor D deficiency downregulated expression of genes related to fatty acid uptake and de novo lipogenesis in the liver. Furthermore, Factor D deficiency reduced the expression of inflammatory factors ( Tnf and Ccl2 ) and fibrosis markers and decreased accumulation of F4/80-positive macrophages. These data suggest that the Factor D deficiency improved hepatic lipid accumulation and hepatic inflammation in HFD-fed mice.

MASP-3 was discovered 15 years ago as the third mannan-binding lectin (MBL)-associated serine protease of the complement lectin pathway. Lacking any verified substrate its role remained ambiguous. MASP-3 was shown to compete with a key lectin pathway enzyme MASP-2 for MBL binding, and was therefore considered to be a negative complement regulator. Later, knock-out mice experiments suggested that MASP-1 and/or MASP-3 play important roles in complement pro-factor D (pro-FD) maturation. However, studies on a MASP-1/MASP-3-deficient human patient produced contradicting results. In normal resting blood unperturbed by ongoing coagulation or complement activation, factor D is present predominantly in its active form, suggesting that resting blood contains at least one pro-FD activating proteinase that is not a direct initiator of coagulation or complement activation. We have recently showed that all three MASPs can activate pro-FD in vitro . In resting blood, however, using our previously evolved MASP-1 and MASP-2 inhibitors we proved that neither MASP-1 nor MASP-2 activates pro-FD. Other plasma proteinases, particularly MASP-3, remained candidates for that function. For this study we evolved a specific MASP-3 inhibitor and unambiguously proved that activated MASP-3 is the exclusive pro-FD activator in resting blood, which demonstrates a fundamental link between the lectin and alternative pathways.

Background Osteoarthritis (OA) is the leading cause of pain worldwide. However, clinical discordance between pain and cartilage damage presents challenges in determining the mechanisms of OA pain, thus creating a need for well-controlled models that probe the separable mechanisms of structural damage and knee pain. We previously identified that deletion of complement factor D (FD) results in increased pressure-pain hyperalgesia despite cartilage protection after destabilization of the medial meniscus (DMM) surgery. However, how these discordant OA phenotypes manifest is not understood. We employed a novel targeted lipidomics approach to elucidate the role of eicosanoids in FD-mediated pain. We hypothesize that the absence of Cfd (FD −/− ) will protect cartilage but cause increased pressure-pain hyperalgesia and eicosanoid dysregulation persisting throughout OA development. Methods Male and female FD −/− and wild-type (WT) mice were challenged with DMM or remained naïve at 16 weeks old. Pressure-pain hyperalgesia was measured every two weeks for 8 weeks post-DMM. A second cohort was evaluated at 2 weeks post-DMM to investigate DMM injury response. Structural damage was scored using the Modified Mankin system. Eicosanoid profiles were characterized via liquid chromatography-mass spectrometry (LC-MS) on serum and synovial fluid samples. Statistical analysis was performed with unpaired t-test or two-way ANOVA with Sidak’s posthoc test, p < 0.05. Results Unlike WT mice, FD −/− mice exhibited no differences in Modified Mankin scores 8 weeks post-DMM in both sexes. As expected, FD −/− and WT hyperalgesia was present at 2 weeks, persisted through 8 weeks, and was not associated with knee structural changes. Despite both sexes exhibiting similar levels of hyperalgesia, eicosanoid profiles differed. Male FD −/− demonstrated greater pain-driving (12-HETE, 13-HODE) and lower pain-driving (15-HETE) and pain-suppressing (14-HDHA) abundances of eicosanoids compared to WT. Paradoxically, female FD −/− exhibited bi-directional differences in pain-suppressive factors (palmitoyl ethanolamide, EPA, 14-HDHA) and lower abundances of pro-inflammatory arachidonic acid compared to WT. Conclusion The absence of Cfd protects cartilage but does not prevent hyperalgesia after DMM. Changes in eicosanoid profiles suggests that loss in FD drives pain acutely and creates a hyperalgesia phenotype early in response to DMM. Eicosanoid profiling is a novel tool to mechanistically determine pain drivers in osteoarthritis.

Malaria and preeclampsia are major pregnancy-related complications that share overlapping complement and inflammation-mediated pathways. Although adipsin has been proposed as a diagnostic biomarker for preeclampsia, its diagnostic performance in the context of concurrent malaria infection remains poorly understood. This study investigated the impact of malaria infection on plasma adipsin levels and evaluated its diagnostic performance for preeclampsia. This case-control study included 200 pregnant women between 20 and 42 weeks of gestation, stratified into four groups: normotensive without malaria, normotensive with malaria, preeclamptic with malaria, and preeclamptic without malaria ( = 50 per group). Plasma adipsin, C3a, C5a, TNF- , IL-6, IL-8 and IFN- were measured using commercial ELISA kits. Malaria infection was confirmed with Giemsa-stained blood smears. Data were analysed using Statistical Package for the Social Sciences (SPSS) Version 27.0. Amongst the participants enrolled, malaria infection was present in 50% and preeclampsia in 50% of the sample. Plasma adipsin levels were significantly elevated in malaria-infected and preeclamptic participants ( < 0.001), with the highest concentrations observed in participants with coexisting preeclampsia and malaria infection. Plasma adipsin showed strong positive correlations with C5a ( = 0.695), IL-6 ( = 0.687), and TNF- ( = 0.645), and moderate correlations with malaria parasite density ( = 0.553), IL-8 ( = 0.475) and C3a ( = 0.437) ( < 0.001 for all). Multivariable regression showed that preeclampsia and malaria independently elevated plasma adipsin levels, with a significant negative interaction between the two conditions ( < 0.001). ROC analysis showed reduced diagnostic specificity for preeclampsia in malaria-infected participants (62.1%, AUC = 0.719, = 0.02) compared with malaria-negative participants (87.9%, AUC = 0.823, < 0.001). infection significantly alters plasma adipsin levels, reducing its diagnostic specificity for preeclampsia. Malaria-adjusted reference thresholds may be necessary when considering adipsin as a biomarker in endemic regions.

Complement factor D (Df) is a serine protease well known for activating the alternative pathway (AP) in mammals by promoting the cleavage of complement component 3 (C3), thus becoming involved in innate defense. In teleost fish, however, the functional mechanisms of Df in the AP and against pathogen infection are far from clear. In the present study, we cloned and characterized the Df gene, CiDf, from grass carp (Ctenopharyngodon idella) and analyzed its function in promoting C3 cleavage and expression changes after grass carp reovirus (GCRV) infection. The open reading frame of CiDf was found to be 753 bp, encoding 250 amino acids with a molecular mass of 27.06 kDa. CiDf harbors a conserved Tryp_SPc domain, with three conserved residues representing the catalytic triad and three conserved binding sites in the substrate specificity pocket. Pairwise alignment showed that CiDf shares the highest identity (96%) and similarity (98%) with Df from Anabarilius grahami. Phylogenetic analysis indicated that CiDf and other fish Dfs formed a distinct evolutionary branch. Similar to most Dfs from other vertebrates, the CiDf gene structure is characterized by four introns and five exons. The incubation of recombinant CiDf protein with grass carp serum significantly increased the C3b content, demonstrating the conserved function of CiDf in the AP in promoting C3 cleavage, similar to Dfs in mammals. CiDf mRNA expression was widely detected in various tissues and levels were relatively higher in the liver, spleen, and intestine of grass carp. During GCRV infection over a 168-hour period, a high level of CiDf mRNA expression in the liver, spleen, and intestine was maintained at 144 and 168 h, suggesting AP activity at the late stage of GCRV infection. Collectively, the above results reveal the conserved structure and function of CiDf and its distinct expression patterns after GCRV infection, which provide a key basis for studying the roles of Df and AP during GCRV infection in the grass carp C. idella.

Lutein is selectively taken up by the primate retina and plays an important role as a filter for harmful blue light and as an antioxidant. Recent studies have shown that lutein has systemic anti-inflammatory properties. Dietary lutein has been associated with reduced circulating levels of inflammatory biomarkers such as CRP and sICAM. Whether lutein also affects activation of the complement system has not yet been addressed and was the purpose of the study described here. Seventy-two subjects with signs of early macular degeneration were randomly assigned to receive either a 10 mg lutein supplement or a placebo during one year. EDTA blood samples were collected at 0, 4, 8 and 12 months. Complement factor D (CFD), a rate limiting component of the alternative pathway of complement activation and the complement activation products C5a and C3d were determined in the plasma samples by ELISA. A significant 0.11 µg/ml monthly decrease in plasma CFD concentration was observed in the lutein group (p<0.001), resulting in a 51% decrease from 2.3 µg/ml at baseline to 1.0 µg/ml at 12 months. The C5a concentration showed a significant 0.063ng/ml monthly decrease in the lutein group (p<0.001) resulting in a 36% decrease from 2.2ng/ml at baseline to 1.6ng/ml at 12 months. The C3d concentration showed a significant 0.19µg/ml monthly decrease in the lutein group (p=0.004) that gave rise to a 9% decrease from 15.4µg/ml at baseline to 14.4µg/ml at 12 months. In the placebo group we found a significant 0.04 µg/ml monthly decrease in plasma CFD concentration, whereas no changes were observed for C5a and C3d. Lutein supplementation markedly decreases circulating levels of the complement factors CFD, C5a and C3d levels, which might allow a simple method to control this inflammatory pathway of the innate immune system.

The aim of this study was to determine whether vitamin D supplementation and maintaining vitamin D sufficiency are associated with changes in inflammatory and metabolic biomarkers in patients with knee osteoarthritis (OA) and vitamin D deficiency. A total of 413 participants with symptomatic knee OA and vitamin D deficiency were enrolled in a randomised, placebo-controlled trial and received 1·25 mg vitamin D3 or placebo monthly for 24 months across two sites. In this post hoc analysis, 200 participants from one site (ninety-four from the placebo group and 106 from the vitamin D group; mean age 63·1 (sd 7·3) years, 53·3 % women) were randomly selected for measurement of serum levels of inflammatory and metabolic biomarkers at baseline and 24 months using immunoassays. In addition, participants were classified into two groups according to serum 25-hydroxyvitamin D (25(OH)D) levels at months 3 and 24: (1) not consistently sufficient (25(OH)D≤50 nmol/l at either month 3 or 24, n 61), and (2) consistently sufficient (25(OH)D>50 nmol/l at both months 3 and 24, n 139). Compared with placebo, vitamin D supplementation had no significant effect on change in serum high-sensitive C-reactive protein, IL-6, IL-8, IL-10, leptin, adiponectin, resistin, adipsin and apelin. Being consistently vitamin D sufficient over 2 years was also not associated with changes in these biomarkers compared with not being consistently sufficient. Vitamin D supplementation and maintaining vitamin D sufficiency did not alter serum levels of inflammatory and metabolic biomarkers over 2 years in knee OA patients who were vitamin D insufficient, suggesting that they may not affect systemic inflammation in knee OA patients.