Key Points Saliva induces bactericidal and DNase resistant NETs in the oral cavity via sialyl LewisX- L-selectin signaling. Disordered homeostasis in the oral cavity may lead to deficient saliva-mediated NETosis.

The complement system is an ancient part of the innate immune system important for both tissue homeostasis and host defense. However, bacteria like Staphylococcus aureus (SA) possess elaborative mechanisms for evading both the complement system and other parts of the immune system. One of these evasive mechanisms—important in causing chronic and therapy resistant infections—is the intracellular persistence in non-immune cells. The objective of our study was to investigate whether persistent intracellular SA infection of epidermal keratinocytes resulted in complement activation. Using fluorescence microscopy, we found that persistent SA, surviving intracellularly in keratinocytes, caused activation of the complement system with formation of the terminal complement complex (TCC) at the cell surface. Skin samples from atopic dermatitis patients analyzed by bacterial culture and microscopy, demonstrated that SA colonization was associated with the presence of intracellular bacteria and deposition of the TCC in epidermis in vivo. Complement activation on keratinocytes with persistent intracellular bacteria was found with sera deficient/depleted of the complement components C1q, Mannan-binding lectin, or complement factor B, demonstrating involvement of more than one complement activation pathway. Viable bacterial counts showed that complement activation at the cell surface initiated cellular responses that significantly reduced the intracellular bacterial burden. The use of an inhibitor of the extracellular signal-regulated kinase (ERK) abrogated the complement-induced reduction in intracellular bacterial load. These data bridge the roles of the complement system in tissue homeostasis and innate immunity and illustrate a novel mechanism by which the complement system combats persistent intracellular bacteria in epithelial cells.

The complement system is activated in response to tissue injury. During wound healing, complement activation seems beneficial in acute wounds but may be detrimental in chronic wounds. We found that the epidermal expression of many complement components was only increased to a minor extent in skin wounds in vivo and in cultured keratinocytes after exposure to supernatant from stimulated mononuclear cells. In contrast, the epidermal expression of complement components was downregulated in ex vivo injured skin lacking the stimulation from infiltrating inflammatory cells but with intact injury-induced epidermal growth factor receptor (EGFR)–mediated growth factor response. In cultured primary keratinocytes, stimulation with the potent EGFR ligand, TGF-α, yielded a significant downregulation of complement component expression. Indeed, EGFR inhibition significantly enhanced the induction of complement components in keratinocytes and epidermis following stimulation with proinflammatory cytokines. Importantly, EGFR inhibition of cultured keratinocytes either alone or in combination with proinflammatory stimulus promoted activation of the complement system after incubation with serum. In keratinocytes treated solely with the EGFR inhibitor, complement activation was dependent on serum-derived C1q, whereas in keratinocytes stimulated with a combination of proinflammatory cytokines and EGFR inhibition, complement activation was found even with C1q-depleted serum. In contrast to human keratinocytes, EGFR inhibition did not enhance complement component expression or cause complement activation in murine keratinocytes. These data demonstrate an important role for EGFR in regulating the expression of complement components and complement activation in human epidermis and keratinocytes and, to our knowledge, identify for the first time a pathway important for the epidermal regulation of complement activation.

Background: Complement plays a pivotal role in the immune response to infection. Several studies demonstrated complement activation in sepsis, yet little is known of the relationship of complement terminal pathway activation and the clinical characteristics of sepsis patients. Therefore, we investigated serum C5, soluble C5b-9 (sC5b-9), and soluble CD59 (sCD59) and their relation to organ failure in sepsis patients in the intensive care unit (ICU). Methods: In this prospective cohort study, all available patients admitted to the adult ICUs between June 2020 and January 2021 were included. Patients were divided into sepsis and nonsepsis groups according to the Sepsis-3 criteria, serum samples from both groups were investigated for the levels of C5, sC5b-9, and sCD59 using commercial sandwich ELISA kits. Results: We analyzed 79 serum samples, 36 were from sepsis patients. We found that sepsis patients had significantly lower C5 (83.6± 28.4 vs 104.4± 32.0 µg/mL, p = 0.004) and higher sCD59 (380.7± 170.5 vs 288.9± 92.5 ng/mL, p = 0.016). sC5b-9, although higher in sepsis patients, did not reach statistical significance (1.5± 0.8 µg/mL vs 1.3± 0.7 µg/mL, p = 0.293). Sepsis patients who died during their ICU stay had significantly higher sCD59 compared to those who survived (437.0 ± 176.7 vs 267.8 ± 79.7 ng/mL, p = 0.003, respectively). Additionally, C5 and sCD59 both correlated to SOFA score in the sepsis group (r s = −0.44, P = 0.007 and = 0.43, P = 0.009, respectively), and a similar correlation was not found in the non-sepsis group. Discussion: In sepsis patients, levels of C5 and sCD59, but not sC5b-9, correlated to the severity of organ damage measured by SOFA. A similar correlation was not found in nonsepsis patients. This indicated that organ damage associated with sepsis led to a more pronounced terminal pathway activation than in non-sepsis patients, it also indicated the potential of using C5 and sCD59 to reflect sepsis severity.

Sepsis is a global health issue that is commonly encountered in the intensive care unit (ICU) and is associated with high morbidity and mortality. Available data regarding sepsis in low- and middle-income countries (LMIC) is lacking compared to higher income countries, especially using updated sepsis definitions. The lack of recent data on sepsis in Jordan prompted us to investigate the burden of sepsis among Jordanian ICU patients. We conducted a prospective cohort study at Jordan University Hospital, a tertiary teaching hospital in the capital, Amman. All adult patients admitted to the adult ICUs between June 2020 and January 2021 were included in the study. Patients’ clinical and demographic data, comorbidities, ICU length of stay (LOS), medical interventions, microbiological findings, and mortality rate were studied. Descriptive and inferential statistics were used to analyse data from patients with and without sepsis. We observed 194 ICU patients during the study period; 45 patients (23.3%) were diagnosed with sepsis using the Sepsis-3 criteria. Mortality rate and median ICU LOS in patients who had sepsis were significantly higher than those in other ICU patients (mortality rate, 57.8% vs. 6.0%, p value < 0.001, resp., and LOS 7 days vs. 4 days, p value < 0.001, resp.). Additionally, sepsis patients had a higher combined number of comorbidities (2.27 ± 1.51 vs. 1.27 ± 1.09, p value < 0.001). The use of mechanical ventilation, endotracheal intubation, and blood transfusions were all significantly more common among sepsis patients. A causative organism was isolated in 68.4% of sepsis patients with a prevalence of Gram-negative bacteria in 77.1% of cases. While the occurrence of sepsis in the ICU in Jordan is comparable to other regions in the world, the mortality rate of sepsis patients in the ICU remains high. Further studies from LMIC are required to reveal the true burden of sepsis globally.