The lectin pathway of complement in pneumococcal infection

Lead Research Organisation: University of Leicester
Department Name: Infection Immunity and Inflammation


This research proposal is based on a strong body of evidence demonstrating the prominent role of an only recently discovered effector arm of the immune system, termed the lectin pathway of complement, in fighting pneumococcal infections. Using an established model of experimental Streptococcus pneumoniae airway infection, the applicants observed that their gene-targeted mouse strain with a total deficiency of the lectin activation pathway is severely compromised in fighting S. pneumoniae infection resulting in a dramatic increase in the severity of pneumococcal disease and mortality, compared to mice with an intact lectin activation pathway. The proposed research aims to define the key components and biochemical mechanisms involved in lectin pathway mediated protection from invasive pneumococcal infection within the immune defense. The availability of a unique combination of mouse strains deficient of either single or several components of the lectin pathway is essential to define the key players in anti-pneumococcal immunity using in vivo models of infection. The in vivo work is supported by an array of sophisticated biochemical analyses using recombinant proteins generated in cell lines to reconstitute and measure lectin pathway functional activity in a test tube. The work programme will also include the analysis of a molecular mechanism used by pathogenic S. pneumoniae strains to evade from the attack of its host s complement system and may explain the essential role of the lectin pathway activation route in fighting infection with these strains. The results of this work programme will have wide implications for the understanding of predispositions to human pneumococcal disease.

Technical Summary

Infections with Streptococcus pneumoniae (the pneumococcus) are the leading cause of bacterial pneumonia, otitis media, bacterial meningitis and septicaemia in children and adults worldwide. Invasive infection is associated with high mortality ranging from 5-35% depending on the site of infection, age and comorbidity. Nevertheless, a high percentage of the population carries commensal S. pneumoniae in the nasopharynx, implying that a healthy immune system is highly effective in keeping invasive infections at bay.
In light of the ever-increasing frequency of antibiotic resistance, it is paramount to understand how the breakdown of the orchestrated response of the innate and adaptive immune system leads to disease.
The complement system plays a key role in fighting S. pneumoniae infection, but the relative contributions of either of the three complement activation pathways and their crosstalk is far from being understood. The proposed work is based on a strong body of evidence showing that the most recently discovered activation pathway of complement, called the lectin activation pathway, is providing a critical degree of protection from pneumococcal infection with a dramatic increase of morbidity and mortality in gene-targeted mice with a total deficiency of lectin pathway functional activity. The severe predisposition of lectin pathway deficient animals for pneumococcal disease is explained by applicants discovery of a so far unknown function of the lectin pathway, which overcomes the ability of pathogenic strains of S. pneumoniae to protect themselves from complement attack by preventing the deposition of the activation fragment of the 4th component of complement (C4b) close to the bacterial surface. This protects bacteria from being opsonised via the classical pathway activation route as C4b is an essential component of the classical pathway C3 convertase C4b2a. The discovery of a novel lectin pathway specific C4-bypass activation route by the applicants, through which S. pneumoniae can be opsonised, is key in understanding the physiological importance of the lectin pathway in anti-pneumococcal immunity.
The proposed project aims:
(i) To determine the synergism and relative contributions of the lectin (LP) and classical (CP) pathways in pneumococcal infection;
(ii) To test the hypothesis that transient or permanent inhibition of the LP will disrupt the normal cellular and soluble inflammatory mediator response to pneumococcal infection;
(iii) To test the hypothesis that ficolin A is the recognition molecule initiating LP activation on pneumococci);
(iv) To elucidate the molecular composition and activation cascade of a novel MASP-2 dependent C4-bypass activation route of the LP.


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Ali YM (2014) Low-dose recombinant properdin provides substantial protection against Streptococcus pneumoniae and Neisseria meningitidis infection. in Proceedings of the National Academy of Sciences of the United States of America

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Almitairi JOM (2018) Structure of the C1r-C1s interaction of the C1 complex of complement activation. in Proceedings of the National Academy of Sciences of the United States of America

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Almitairi JOM (2018) Reply to Mortensen et al.: The zymogen form of complement component C1. in Proceedings of the National Academy of Sciences of the United States of America

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Asgari E (2014) Mannan-binding lectin-associated serine protease 2 is critical for the development of renal ischemia reperfusion injury and mediates tissue injury in the absence of complement C4. in FASEB journal : official publication of the Federation of American Societies for Experimental Biology

Title Mouse specific rat anti MASP-2 antibodies 
Description We have established 3 different hybridoma lines producing mouse specific rat anti MASP-2 antibodies 
Type Of Material Antibody 
Year Produced 2009 
Provided To Others? Yes  
Impact We have developed the first ELISA measuring MASP-2 serum levels in the mouse. This can be used to monitor the state of lectin pathway activation during -for example- infection. 
Description The lectin activation pathway of complement critically contributes to the pathophysiology of multi-organ failure 
Organisation University of Regensburg
Department Institute of Immunology
Country Germany 
Sector Academic/University 
PI Contribution We developed the concept, provided the gene-targeted mouse models and inhibitors, and analysed the experimental outcome.
Collaborator Contribution We have studied the phenotype of MASP-2 deficiency in polymicrobial peritonitis and found that MASP-2 deficient mice are severely compromised in their ability to fight polymicrobial peritonitis resulting in several Logs higher bacterial CFUs recovered from peritoneal lavage of MASP-2-/- mice compared to their MASP-2 +/+ littermates. However, both lines show no differences in mortality. We have searched for a physiological explanation of this phenomenon and found that MASP-2 -/- mice are significantly protected from septic shock mediated multi-organ failure and do not show an LPS/TNF driven Shwartzman reaction in vivo. This new phenotypic presentation of lectin pathway deficiency implies a therapeutic benefit using lectin pathway inhibitors to prevent agglutination and closure of the low blood pressure circulatory system resulting in septic shock mediated multi-organ failure, the main cause of death in end stage infectious disease.
Impact This collaboration has lead to a high quality manuscript presently submitted for publication.
Start Year 2009