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Background Serological assays for Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) have roles in seroepidemiology, convalescent plasma-testing, antibody durability and vaccine studies. Currently, SARS-CoV-2 serology is performed using serum/plasma collected by venepuncture. Dried blood spot (DBS) testing offers significant advantages as it is minimally invasive, avoids venepuncture with specimens being mailed to the laboratory. Methods A pathway utilizing a newborn screening laboratory infrastructure was developed using an enzyme-linked immunosorbent assay to detect IgG antibodies against the receptor-binding domain of the SARS-CoV-2 spike protein in DBS specimens. Paired plasma and DBS specimens from SARS-CoV-2 antibody-positive and -negative subjects and polymerase chain reaction positive subjects were tested. DBS specimen stability, effect of blood volume and punch location were also evaluated. Results DBS specimens from antibody-negative ( n = 85) and -positive ( n = 35) subjects and polymerase chain reaction positive subjects ( n = 11) had a mean (SD; range) optical density (OD) of 0.14 (0.046; 0.03–0.27), 0.98 (0.41; 0.31–1.64) and 1.12 (0.37; 0.49–1.54), respectively. An action value OD >0.28 correctly assigned all cases. The weighted Deming regression for comparison of the DBS and the plasma assay yielded: y = 0.004041 + 1.005 x, r = 0.991, Sy/ x 0.171, n = 82. Extraction efficiency of antibodies from DBS specimens was >99%. DBS specimens were stable for at least 28 days at ambient room temperature and humidity. Conclusions SARS-CoV-2 IgG receptor-binding domain antibodies can be reliably detected in DBS specimens. DBS serological testing offers lower costs than either point of care or serum/plasma assays that require patient travel, phlebotomy and hospital/clinic resources; the development of a DBS assay may be particularly important for resource poor settings.

Introduction Coagulation and complement systems are simultaneously activated at sites of tissue injury, leading to thrombin generation and opsonisation with C3b. Thrombomodulin (TM) is a cell-bound regulator of thrombin activation, but can also enhance the regulatory activity of complement factor H (FH), thus accelerating the degradation of C3b into inactive iC3b. Objectives This study sought to determine the biophysical interaction affinities of two recombinant TM analogs with thrombin, FH and C3b in order to analyze their ability to regulate serum complement activity. Methods Surface plasmon resonance (SPR) analysis was used to determine binding affinities of TM analogs with FH and C3b, and compared to thrombin as positive control. The capacity of the two recombinant TM analogs to regulate complement in serum was tested in standard complement hemolytic activity assays. Results SPR analysis showed that both TM analogs bind FH and C3b-Factor H with nanomolar and C3b with micromolar affinity; binding affinity for its natural ligand thrombin was several fold higher than for FH. At a physiological relevant concentration, TM inhibits complement hemolytic activity in serum via FH dependent and independent mechanisms. Conclusions TM exhibits significant binding affinity for complement protein FH and C3b-FH complex and its soluble form is capable at physiologically relevant concentrations of inhibiting complement activation in serum.

SUMMARYTwo sensitive sandwich ELISAs based on monoclonal antibodies directed to native C6 and C7 allowed the detection and quantitation of these complement proteins in 20 out of 37 serum samples from individuals who had previously been classified as deficient in these proteins as assessed by immunochemical and/or functional assays. Furthermore, serum from four C6-deficient and one combined C6-/C7-deficient individual showed an increase in the terminal complement complex (TCC) and a decrease in native C6 and C7 after complement activation as assayed by specific ELISAs. Despite their (incomplete) deficiencies, these individuals therefore possess functionally active terminal complement proteins with respect to their ability to generate the TCC. As these individuals have no history of a susceptibility to neisserial infections, even low concentrations of functionally active C6 and C7 may provide sufficient protection against those micro-organisms whose destruction requires TCC formation.

Complement activation is a crucial early event in Wallerian degeneration. In this study we show that treatment of rats with soluble complement receptor 1 (sCR1), an inhibitor of all complement pathways, blocked both systemic and local complement activation after crush injury of the sciatic nerve. Deposition of membrane attack complex (MAC) in the nerve was inhibited, the nerve was protected from axonal and myelin breakdown at 3 days after injury, and macrophage infiltration and activation was strongly reduced. We show that both classical and alternative complement pathways are activated after acute nerve trauma. Inhibition of the classical pathway by C1 inhibitor (Cetor) diminished, but did not completely block, MAC deposition in the injured nerve, blocked myelin breakdown, inhibited macrophage infiltration, and prevented macrophage activation at 3 days after injury. However, in contrast to sCR1 treatment, early signs of axonal degradation were visible in the nerve, linking MAC deposition to axonal damage. We conclude that sCR1 protects the nerve from early axon loss after injury and propose complement inhibition as a potential therapy for the treatment of diseases in which axon loss is the main cause of disabilities.

The human complement regulator CD55 is a key molecule protecting self-cells from complement-mediated lysis. X-ray diffraction and analytical ultracentrifugation data reveal a rod-like arrangement of four short consensus repeat (SCR) domains in both the crystal and solution. The stalk linking the four SCR domains to the glycosylphosphatidylinositol anchor is extended by the addition of 11 highly charged O-glycans and positions the domains an estimated 177 Å above the membrane. Mutation mapping and hydrophobic potential analysis suggest that the interaction with the convertase, and thus complement regulation, depends on the burial of a hydrophobic patch centered on the linker between SCR domains 2 and 3.

Patients whose blood is deficient in the terminal component of complement have an increased susceptibility to meningococcal infection. However, mortality from meningococcal infection is lower in these patients than in immunocompetent subjects. We studied a C6-deficient patient with meningococcal sepsis who received fresh frozen plasma (FFP). The patient's initial plasma endotoxin, C6, and terminal-complement-complex concentrations were low, but rose sharply after treatment with FFP. Samples of the patient's serum taken shortly after admission did not cause endotoxin release from Escherichia coli J5 in vitro, but endotoxin-releasing activity was restored in serum samples taken after infusion of FFP. It is possible that C6-deficient patients have reduced mortality from meningococcal infection because their serum cannot cause acute release of endotoxin from the invading organism and extensive tissue damage is thus avoided.

Thyroid cells are exposed to complement attack in Graves' disease and Hashimoto's thyroiditis, but are resistant to killing by homologous complement. We have examined the effects of sublethal complement attack on thyroid cells in vitro. Extracellular reactive oxygen metabolites were produced and prostaglandin E2, interleukin-1 alpha, and interleukin-6 were released after complement attack. Cells pretreated with interferon-gamma and interleukin-1 alpha, which increase expression of CD59, were more resistant to these effects of complement. Conversely, blockade of CD59 with monoclonal antibody increased complement-mediated oxygen radical production and release of prostaglandin E2, interleukin-1 alpha, and interleukin-6. The antithyroid drugs methimazole and propylthiouracil abolished or reduced oxygen radical production by complement-attacked thyroid cells and reduced cytokine release. These results suggest that sublethal complement attack in autoimmune thyroid diseases exacerbates tissue injury by causing thyroid cells to release potent phlogistic mediators, although some degree of protection may be afforded in vivo by cytokine-mediated upregulation of CD59. Antithyroid drugs, concentrated within thyroid cells, will prevent the release of these inflammatory molecules, which may in turn explain the amelioration of thyroiditis and remission of Graves' disease after such treatment.

An immunoradiometric assay was used to measure the concentration of the terminal component of complement (C9) in cerebrospinal fluid (CSF) and plasma from 35 patients with multiple sclerosis and 55 controls with other neurological diseases. There was a highly significant reduction in cerebrospinal fluid C9 concentration in patients with multiple sclerosis (0.26 +/- 0.02 microgram/ml) compared with controls (1.52 +/- 0.20 micrograms/ml; p less than 0.0005). As a single protein measurement C9 seemed to be more useful as an aid to clinical diagnosis than CSF IgG; the C9 index was also a better discriminator than IgG index between the two groups of patients. Reduced CSF C9 concentration in patients with multiple sclerosis implies C9 consumption due to formation of membrane attack complexes, which could mediate myelin damage and cause more widespread but reversible loss of function, accounting for the transient symptoms characteristic of the disease.

Disturbances in intracellular Ca2+ are known to be important in cell injury caused by a wide range of toxic factors. The complement system is a major effector of immune damage in vivo, and is known to be involved in the pathogenesis of many immune diseases. We present here evidence that the potentially lethal membrane attack complex of complement causes a rapid increase in intracellular free Ca2+ concentration before any other detectable biochemical changes in the cell. In nucleated cells the increased intracellular free Ca2+ concentration initially stimulates recovery processes, allowing the cell to escape mild complement attack and also activates the production of inflammatory mediators, which may amplify an ongoing inflammatory response. More severe complement membrane attack causes a more rapid rise in intracellular free Ca2+ concentration allowing a threshold to be breached above which recovery processes are overwhelmed, and cell death occurs. The importance of non-lytic effects and recovery processes mediated by Ca2+, and the molecular basis of these effects are discussed, and the hypothesis proposed that the cell-injuring effects of other "pore-forming" toxins are also caused by increases in intracellular free Ca2+.