Complement and properidin deficiencies in meningococcal disease

The hijackers guide to escaping complement: Lessons learned from pathogens

Pathogens that invade the human host are confronted by a multitude of defence mechanisms aimed at preventing colonization, dissemination and proliferation. The most frequent outcome of this interaction is microbial elimination, in which the complement system plays a major role. Complement, an essential feature of the innate immune machinery, rapidly identifies and marks pathogens for efficient removal. Consequently, this creates a selective pressure for microbes to evolve strategies to combat complement, permitting host colonization and access to resources. All successful pathogens have developed mechanisms to resist complement activity which are intimately aligned with their capacity to cause disease. In this review, we describe the successful methods various pathogens use to evade complement activation, shut down inflammatory signalling through complement, circumvent opsonisation and override terminal pathway lysis. This review summarizes how pathogens undermine innate immunity: 'The Hijackers Guide to Complement'.

Microbiological Diagnosis of Skeletal System Infections in Children

BACKGROUND

If not timely diagnosed and adequately treated, skeletal system infections in children may result in severe and permanent disability. Prompt identification of the etiology of the disease and determination of its antibiotic susceptibility are crucial for the successful management of septic arthritis, osteomyelitis, and spondylodiscitis. However, the bacteriological diagnosis of these infections has been traditionally limited by the low yield of conventional cultures and, on average, one-third of cases of pediatric joint and bone infections remained unconfirmed.

OBJECTIVE

To review the medical literature to summarize the current approach diagnosing the pediatric skeletal system infections.

METHODS

The relevant publications for the last three decades were reviewed.

RESULTS

In recent years, the detection of skeletal system pathogens has been revolutionized by the use of improved laboratory methods, including seeding of synovial fluid and bone exudates into blood culture vials, and the development and implementation of sensitive nucleic acid amplification assays. These advances have resulted in the recognition of Kingella kingae as the predominant etiology of hematogenous infections of bones, joints, intervertebral discs and tendon sheaths in children aged 6-48 months, and reduced the fraction of culture-negative osteoarthritis.

CONCLUSION

As the exudate and tissue samples obtained from young children with skeletal system infections are frequently insufficient for a comprehensive laboratory workup, physicians should take in consideration the patient's age, predisposing medical conditions and possible exposure to zoonotic organisms, and use a judicious combination of Gram's stain, culture on blood culture vials, and molecular tests to maximize the microbiological diagnosis of these diseases.

Properdin: a tightly regulated critical inflammatory modulator

The complement alternative pathway is a powerful arm of the innate immune system that enhances diverse inflammatory responses in the human host. Key to the effects of the alternative pathway is properdin, a serum glycoprotein that can both initiate and positively regulate alternative pathway activity. Properdin is produced by many different leukocyte subsets and circulates as cyclic oligomers of monomeric subunits. While the formation of non‐physiological aggregates in purified properdin preparations and the presence of potential properdin inhibitors in serum have complicated studies of its function, properdin has, regardless, emerged as a key player in various inflammatory disease models. Here, we review basic properdin biology, emphasizing the major hurdles that have complicated the interpretation of results from properdin‐centered studies. In addition, we elaborate on an emerging role for properdin in thromboinflammation and discuss the potential utility of properdin inhibitors as long‐term therapeutic options to treat diseases marked by increased formation of platelet/granulocyte aggregates. Finally, we describe the interplay between properdin and the alternative pathway negative regulator, Factor H, and how aiming to understand these interactions can provide scientists with the most effective ways to manipulate alternative pathway activation in complex systems.

Tumour cell conditioned medium reveals greater M2 skewing of macrophages in the absence of properdin

Introduction

The tumour microenvironment is shaped by the interaction of immune, non immune, and tumour cells present in close proximity. Tumour cells direct the development of a locally immune suppressed state, affecting the activity of anti tumour T cells and preparing the escape phase of tumour development. Macrophages in the tumour typically develop into so‐called tumour associated macrophages with a distinct profile of activities which lead to a reduction in inflammation and antigen presentation. The direct impact of tumour cell conditioned medium on the activity profile of macrophages in dependence of their complement component expression has not yet been investigated.

Methods

In our in vitro study, macrophages differentiated from bone marrows of properdin deficient and wildtype mice were stimulated with conditioned medium of a syngeneic tumour cell line, B16F10, a mouse melanoma subline.

Results

In comparison with macrophages from wildtype mice, those from congenic properdin deficient mice showed skewing towards M2 profile, encompassing mRNA expression for genes involved in arginine metabolism, production of type 2 cytokines, and relatively lower surface expression of molecules needed for antigen presentation.

Conclusions

These data suggest that properdin insufficiency promotes a tumour environment that helps the tumour evade the immune response.

Properdin-Mediated C5a Production Enhances Stable Binding of Platelets to Granulocytes in Human Whole Blood

Enhanced levels of platelet/granulocyte aggregates (PGAs) are found in patients suffering from many different inflammatory vascular diseases, and their formation in animal models of vascular disease is associated with increased thromboinflammation and worsened outcomes. The complement system, a part of the innate immune system, influences PGA formation, but the mechanisms for its effects are unknown. In this study, we have defined complement-mediated mechanisms that enhance PGA formation in human whole blood stimulated with thrombin receptor-activating peptide (TRAP) using ex vivo flow cytometry assays. We demonstrate that physiological properdin, a positive regulator of complement alternative pathway activity, increases PGA formation when added to TRAP-stimulated blood. All physiological properdin forms increase PGA formation, but properdin tetramers are the most efficient at increasing complement activity and PGA formation. Inhibition of endogenous properdin, either circulating in the blood or produced locally by leukocytes, impairs TRAP-mediated PGA formation to the same level as specific inhibition of either the alternative or classical pathway. Additionally, blocking the interaction of C5a with its cellular receptor prevents properdin-mediated increases in PGA formation. Adding either properdin tetramers or C5a to whole blood increases CD11b expression on granulocytes, and this increase is prevented by blockade of the C5a-C5a receptor axis. Finally, we demonstrate that the effects of properdin on PGA formation are tightly regulated by Factor H. Cumulatively, our data indicate that properdin enhances PGA formation via increased production of C5a, and that inhibition of properdin function has therapeutic potential to limit thromboinflammation in diseases characterized by increased PGA formation.

Recognition of Neisseria meningitidis by the long pentraxin PTX3 and its role as an endogenous adjuvant

Long pentraxin 3 (PTX3) is a non-redundant component of the humoral arm of innate immunity. The present study was designed to investigate the interaction of PTX3 with Neisseria meningitidis. PTX3 bound acapsular meningococcus, Neisseria-derived outer membrane vesicles (OMV) and 3 selected meningococcal antigens (GNA0667, GNA1030 and GNA2091). PTX3-recognized microbial moieties are conserved structures which fulfil essential microbial functions. Ptx3-deficient mice had a lower antibody response in vaccination protocols with OMV and co-administration of PTX3 increased the antibody response, particularly in Ptx3-deficient mice. Administration of PTX3 reduced the bacterial load in infant rats challenged with Neisseria meningitidis. These results suggest that PTX3 recognizes a set of conserved structures from Neisseria meningitidis and acts as an amplifier/endogenous adjuvant of responses to this bacterium.

Primary immunodeficiency diseases worldwide: more common than generally thought

PURPOSE

Primary immunodeficiency diseases (PIDs) comprise at least 176 hereditary disorders that are thought to be individually and collectively rare. The actual prevalence and incidence of PIDs remains unclear, but recent epidemiologic studies have suggested that PIDs are more common than generally thought. Based on these studies, we attempted to estimate the worldwide prevalence and incidence of PIDs.

METHODS

Using data from registries and two recent epidemiologic surveys estimating the frequencies of PIDs, we extrapolated the frequencies reported for certain countries to the populations of continents and of the world.

RESULTS

Our upper estimates suggest that six million people may be living with a PID worldwide, whereas only 27,000-60,000 have been identified to date (all national registries and the Jeffrey Modell Centers Network, respectively). For Europe, our upper estimate was 638,000 cases, and 15,052 cases are currently registered (2.27 %). In Africa, up to 902,631 people may have a PID, whereas only 1,016 cases are currently registered. We also found that PIDs were prevalent not only in children, but also in adults, who were strongly underrepresented in registries.

CONCLUSION

Specific, dedicated epidemiologic studies are required, to obtain more realistic statistics for PIDs and to increase the awareness of physicians and public health systems about these diseases. Furthermore, the field of PIDs is continually growing, and this is likely to lead to a revision of the definition of these conditions, potentially increasing estimates of their impact on both adults and children, at the population level.

A novel mutation W388X underlying properdin deficiency in a Finnish family

Properdin deficiency is a rare immunological disorder inherited as an X-chromosomal recessive trait. Properdin deficiency poses a significant risk for severe meningococcal infections. About 20 mutations have been reported to underlie properdin deficiency. Here we report a large Finnish family with a novel mutation in the properdin gene (CFP). Based on the total absence of properdin activity in a 14-year-old male patient with an infection resembling meningococcal bacteraemia, the coding region and splice sites of the gene were sequenced. The mutation is located in exon 9 and changes guanine to adenine at nucleotide 1164 (c.1164G>A) that causes tryptophan to change to a premature stop codon (W388X). The mother of the patient was shown to be a carrier of the mutation. In total, the mutation was identified in six females and three young males in the family. The mutation must be inherited from the grandfather who had died of an unknown infectious disease. This is the first mutation of the properdin gene identified in Finland.

Host-pathogen interactions revealed by human genome-wide surveys

Genome-wide association studies (GWAS) have now convincingly shown that the diverse outcomes (such as the resolution of infection, clinical deterioration to severe disease, or progression from acute infection to persistent infection) that occur following microbial infection can be at least partly explained by human genetic variation. Unbiased whole-genome approaches have revealed unprecedentedly robust associations between genetic markers and susceptibility to disease, providing clear insights into our understanding of infectious disease biology by revealing the crucial host-pathogen interaction sites. Further work characterizing both the host causative variations and pathogenic microbial strains with distinct host interactions and disease outcomes is now required to provide potential new intervention strategies.

Human TLRs and IL-1Rs in host defense: natural insights from evolutionary, epidemiological, and clinical genetics

Toll-like receptors (TLRs) and interleukin-1 receptors (IL-1Rs) have TIR intracellular domains that engage two main signaling pathways, via the TIR-containing adaptors MyD88 (which is not used by TLR3) and TRIF (which is used only by TLR3 and TLR4). Extensive studies in inbred mice in various experimental settings have attributed key roles in immunity to TLR- and IL-1R-mediated responses, but what contribution do human TLRs and IL-1Rs actually make to host defense in the natural setting? Evolutionary genetic studies have shown that human intracellular TLRs have evolved under stronger purifying selection than surface-expressed TLRs, for which the frequency of missense and nonsense alleles is high in the general population. Epidemiological genetic studies have yet to provide convincing evidence of a major contribution of common variants of human TLRs, IL-1Rs, or their adaptors to host defense. Clinical genetic studies have revealed that rare mutations affecting the TLR3-TRIF pathway underlie herpes simplex virus encephalitis, whereas mutations in the TIR-MyD88 pathway underlie pyogenic bacterial diseases in childhood. A careful reconsideration of the contributions of TLRs and IL-1Rs to host defense in natura is required.

Characterization of diverse subvariants of the meningococcal factor H (fH) binding protein for their ability to bind fH, to mediate serum resistance, and to induce bactericidal antibodies

Neisseria meningitidis is a commensal of the human nasopharynx but is also a major cause of septicemia and meningitis. The meningococcal factor H binding protein (fHbp) binds human factor H (fH), enabling downregulation of complement activation on the bacterial surface. fHbp is a component of two serogroup B meningococcal vaccines currently in clinical development. Here we characterize 12 fHbp subvariants for their level of surface exposure and ability to bind fH, to mediate serum resistance, and to induce bactericidal antibodies. Flow cytometry and Western analysis revealed that all strains examined expressed fHbp on their surface to different extents and bound fH in an fHbp-dependent manner. However, differences in fH binding did not always correlate with the level of fHbp expression, indicating that this is not the only factor affecting the amount of fH bound. To overcome the issue of strain variability in fHbp expression, the MC58ΔfHbp strain was genetically engineered to express different subvariants from a constitutive heterologous promoter. These recombinant strains were characterized for fH binding, and the data confirmed that each subvariant binds different levels of fH. Surface plasmon resonance revealed differences in the stability of the fHbp-fH complexes that ranged over 2 orders of magnitude, indicating that differences in residues between and within variant groups can influence fH binding. Interestingly, the level of survival in human sera of recombinant MC58 strains expressing diverse subvariants did not correlate with the level of fH binding, suggesting that the interaction of fHbp with fH is not the only function of fHbp that influences serum resistance. Furthermore, cross-reactive bactericidal activity was seen within each variant group, although the degree of activity varied, suggesting that amino acid differences within each variant group influence the bactericidal antibody response.

Shared pathways to infectious disease susceptibility?

The recent advent of genomic approaches for association testing is starting to enable a more comprehensive understanding of the role of human immune response in determining infectious disease outcomes. Progressing from traditional linkage approaches using microsatellite markers to high-resolution genome-wide association scans, these new approaches are leading to the robust discovery of a large number of disease susceptibility genes and the beginnings of an appreciation of their connections. In this commentary, we discuss how this technology development has led to increasingly complex and common infectious diseases being unraveled, and how this is starting to dissect pathogen-specific human responses. Intriguingly, these still preliminary findings suggest that pathogen innate detection mechanisms may not be as shared among diseases as immune response mechanisms.

Molecular mechanisms of complement evasion: learning from staphylococci and meningococci

The complement system is a crucial component of the innate immune response in humans. Recent studies in Staphylococcus aureus and Neisseria meningitidis have revealed how these bacteria escape complement-mediated killing. In addition, new structural data have provided detailed insights into the molecular mechanisms of host defence mediated by the complement system and how bacterial proteins interfere with this process. This information is fundamental to our understanding of bacterial pathogenesis and may facilitate the design of better vaccines.

Genetic and environmental influences on risk of death due to infections assessed in Danish twins, 1943-2001

Genetic differences have been proposed to play a strong role in risk of death from infectious diseases. The study base of 44,005 included all same-sex twin pairs born in 1870-2001, with both twins alive on January 1, 1943, or those born thereafter. Cause of death was obtained from the Danish Cause of Death Register and was available for 18,359 deaths. The authors classified death due to infections by 3 definitions (narrow, broader, and broadest) and calculated concordance rates for same-sex monozygotic and dizygotic twin pairs. Heritability was estimated by using structural equation models. When the 3 definitions were applied, 211 (1.1%), 1,089 (5.9%), and 2,907 (15.8%) deaths, respectively, were due to infections. The probandwise concordance rates for monozygotic twin pairs were consistently higher than for dizygotic twin pairs regardless of the definition (9% vs. 0% (P = 0.04), 10% vs. 3% (P < 0.01), and 19% vs. 15% (P = 0.07), respectively). For the broader and broadest definitions, heritability was 40% (95% confidence interval: 12, 50) and 19% (95% confidence interval: 3, 35), respectively. The concordance rates were generally low, and, although a genetic influence on the risk of death from infectious diseases could be demonstrated, the absolute effect of the genetic component on mortality was small.

Primary immunodeficiencies of protective immunity to primary infections

The vast majority of primary immunodeficiencies (PIDs) predispose affected individuals to recurrent or chronic infectious diseases, because they affect protective immunity to both primary and secondary or latent infections. We discuss here three recently described groups of PIDs that seem to impair immunity to primary infections without compromising immunity to secondary and latent infections. Patients with mutations in IL12B or IL12RB1 typically present mycobacterial disease in childhood with a favorable progression thereafter. Cross-protection between mycobacterial infections has even been observed. Patients with mutations in IRAK4 or MYD88 suffer from pyogenic bacterial diseases, including invasive pneumococcal diseases in particular. These diseases often recur, although not always with the same serotype, but the frequency of these recurrences tails off, with no further infections observed from adolescence onwards. Finally, mutations in UNC93B1 and TLR3 are associated with childhood herpes simplex encephalitis, which strikes only once in most patients, with almost no recorded cases of more than two bouts of this disease. Unlike infections in patients with other PIDs, the clinical course of which typically deteriorates with age even if appropriate treatment is given, the prognosis of patients with these three newly described PIDs tends to improve spontaneously with age, provided, of course, that the initial infection is properly managed. In other words, although life-threatening in early childhood, these new PIDs are associated with a favorable outcome in adulthood. They provide proof-of-principle that infectious diseases of childhood striking only once may result from single-gene inborn errors of immunity.

Influence of serogroup B meningococcal vaccine antigens on growth and survival of the meningococcus in vitro and in ex vivo and in vivo models of infection

A novel vaccine against serogroup B meningococcal disease - containing a combination of protein antigens identified by reverse vaccinology: fHBP fused to GNA2091, GNA2132 fused to GNA1030, and NadA - is currently in Phase III clinical trials. In order to determine the role of these antigens in the growth, survival and fitness of the meningococcus, we generated a mutant lacking the expression of all five protein antigens (5KO), a mutant lacking the three main antigens (fHBP, GNA2132 and NadA; 3KO), as well as strains lacking the single antigens. Our results show that abrogation of expression of these antigens in Neisseria meningitidis results in reduced growth in vitro, increased sensitivity of the bacterium to stresses it may encounter in the host, as well as reduced fitness in ex vivo models of infection and in an in vivo infant rat competitive index assay. These results support a multivalent vaccine approach, which was undertaken to strengthen the protective activity of the vaccine antigens, increase the breadth of MenB strains targeted by the vaccine, and limit the potential for selection of vaccine escape mutants.

Mechanisms of avoidance of host immunity by Neisseria meningitidis and its effect on vaccine development

Neisseria meningitidis remains an important cause of severe sepsis and meningitis worldwide. The bacterium is only found in human hosts, and so must continually coexist with the immune system. Consequently, N meningitidis uses multiple mechanisms to avoid being killed by antimicrobial proteins, phagocytes, and, crucially, the complement system. Much remains to be learnt about the strategies N meningitidis employs to evade aspects of immune killing, including mimicry of host molecules by bacterial structures such as capsule and lipopolysaccharide, which poses substantial problems for vaccine design. To date, available vaccines only protect individuals against subsets of meningococcal strains. However, two promising vaccines are currently being assessed in clinical trials and appear to offer good prospects for an effective means of protecting individuals against endemic serogroup B disease, which has proven to be a major challenge in vaccine research.

Recent advances in primary immunodeficiencies: identification of novel genetic defects and unanticipated phenotypes

Primary immunodeficiencies (PIDs) have traditionally been defined according to their immunologic phenotype. Far from being concluded, the search for human genes that, when mutated, cause PID is actively being pursued. During the last year, four novel genetic defects that cause severe combined immunodeficiency and severe congenital neutropenia have been identified. At the same time, the immunologic definition of primary immunodeficiencies has been expanded by the recognition that genetic defects affecting innate immunity may result in selective predisposition to certain infections, such as mycobacterial disease, herpes simplex encephalitis, and invasive pneumococcal infections. Studies of genetically determined susceptibility to infections have recently shown that immunologic defects may also account for novel infectious phenotypes, such as malaria or leprosy. Finally, a growing body of evidence indicates that primary immunodeficiencies may present with a noninfectious clinical phenotype that may be restricted to single organs, as in the case of atypical hemolytic uremic syndrome or pulmonary alveolar proteinosis. Overall, these achievements highlight the importance of human models, which often differ from the corresponding animal models.

Factor H-binding protein is important for meningococcal survival in human whole blood and serum and in the presence of the antimicrobial peptide LL-37

Factor H-binding protein (fHBP; GNA1870) is one of the antigens of the recombinant vaccine against serogroup B Neisseria meningitidis, which has been developed using reverse vaccinology and is the basis of a meningococcal B vaccine entering phase III clinical trials. Binding of factor H (fH), an inhibitor of the complement alternative pathway, to fHBP enables N. meningitidis to evade killing by the innate immune system. All fHBP null mutant strains analyzed were sensitive to killing in ex vivo human whole blood and serum models of meningococcal bacteremia with respect to the isogenic wild-type strains. The fHBP mutant strains of MC58 and BZ83 (high fHBP expressors) survived in human blood and serum for less than 60 min (decrease of >2 log(10) CFU), while NZ98/254 (intermediate fHBP expressor) and 67/00 (low fHBP expressor) showed decreases of >1 log(10) CFU after 60 to 120 min of incubation. In addition, fHBP is important for survival in the presence of the antimicrobial peptide LL-37 (decrease of >3 log(10) CFU after 2 h of incubation), most likely due to electrostatic interactions between fHBP and the cationic LL-37 molecule. Hence, the expression of fHBP by N. meningitidis strains is important for survival in human blood and human serum and in the presence of LL-37, even at low levels. The functional significance of fHBP in mediating resistance to the human immune response, in addition to its widespread distribution and its ability to induce bactericidal antibodies, indicates that it is an important component of the serogroup B meningococcal vaccine.

From infectious diseases to primary immunodeficiencies

The field of primary immunodeficiencies has expanded, thanks to the exploration of novel clinical phenotypes and their connection with morbid genotypes, and the subsequent exploration of new patients who have known primary immunodeficiency-defining clinical phenotypes and their connection with novel morbid genotypes. This two-way process is becoming increasingly active, particularly for patients who have infectious diseases in whom the underlying immunologic and genetic causes remain mostly unexplained. The authors review how the exploration of children who have clinical infectious diseases caused by mycobacteria, pneumococcus, and herpes simplex virus recently led to the description of three new groups of primary immunodeficiencies. These three examples justify the continuation of the genetic exploration of novel infectious phenotypes and novel patients who have infections. This challenging process will eventually reap its rewards, to the benefit of patients and their families.

Revisiting human primary immunodeficiencies

Human primary immunodeficiencies (PIDs) are often thought to be confined to a few rare, familial, monogenic, recessive traits impairing the development or function of one or several leucocyte subsets and resulting in multiple, recurrent, opportunistic and fatal infections in infancy. We highlight here the rapidly growing number of exceptions to each of these conventional qualifications. Indeed, bona fide PIDs include common and sporadic illnesses and may present as dominant, or even polygenic traits; their pathogenesis may involve non haematopoietic cells, and they may result in single episode of illness, with a single or multiple morbid phenotypes, some of which may involve infection, in otherwise healthy adults. We need to increase awareness of the multitude of clinical presentations of human PIDs considerably and rapidly in the medical community. Human PIDs should be considered in a wide range of clinical situations.

Stages of meningococcal sepsis simulated in vitro, with emphasis on complement and Toll-like receptor activation

The clinical presentation of meningococcal disease is closely related to the number of meningococci in the circulation. This study aimed to examine the activation of the innate immune system after being exposed to increasing and clinically relevant concentrations of meningococci. We incubated representative Neisseria meningitidis serogroup B (ST-32) and serogroup C (ST-11) strains and a lipopolysaccharide (LPS)-deficient mutant (the 44/76 lpxA mutant) in human serum and whole blood and measured complement activation and cytokine secretion and the effect of blocking these systems. HEK293 cells transfected with Toll-like receptors (TLRs) were examined for activation of NF-kappaB. The threshold for cytokine secretion and activation of NF-kappaB was 10(3) to 10(4) meningococci/ml. LPS was the sole inflammation-inducing molecule at concentrations up to 10(5) to 10(6) meningococci/ml. The activation was dependent on TLR4-MD2-CD14. Complement contributed to the inflammatory response at >or=10(5) to 10(6) meningococci/ml, and complement activation increased exponentially at >or=10(7) bacteria/ml. Non-LPS components initiated TLR2-mediated activation at >or=10(7) bacteria/ml. As the bacterial concentration exceeded 10(7)/ml, TLR4 and TLR2 were increasingly activated, independent of CD14. In this model mimicking human disease, the inflammatory response to N. meningitidis was closely associated with the bacterial concentration. Therapeutically, CD14 inhibition alone was most efficient at a low bacterial concentration, whereas addition of a complement inhibitor may be beneficial when the bacterial load increases.

Immunology in natura: clinical, epidemiological and evolutionary genetics of infectious diseases

The field of human genetics of infectious diseases defines the genes and alleles rendering individuals (clinical genetics) and populations (epidemiological genetics) vulnerable to infection, and studies those selected by previous infections (evolutionary genetics). These disciplines--clinical, epidemiological and evolutionary genetics-delineate the redundant and nonredundant functions of host defense genes for past and present survival in natura--in natural ecosystems governed by natural selection. These disciplines, in other words, assess the ecologically relevant and evolutionarily selected roles of human genes and alleles in protective immunity to diverse and evolving microorganisms. The genetic dissection of human immunity to infection in natura provides unique immunological insight, making it an indispensable complement to experimental immunology in vitro and in vivo in plants and animals.

Primary immunodeficiencies: a field in its infancy

A paradigm shift is occurring in the field of primary immunodeficiencies, with revision of the definition of these conditions and a considerable expansion of their limits. Inborn errors of immunity were initially thought to be confined to a few rare, familial, monogenic, recessive traits impairing the development or function of one or several leukocyte subsets and resulting in multiple, recurrent, opportunistic, and fatal infections in infancy. A growing number of exceptions to each of these conventional qualifications have gradually accumulated. It now appears that most individuals suffer from at least one of a multitude of primary immunodeficiencies, the dissection of which is helping to improve human medicine while describing immunity in natura.

Meningococcal disease associated with an acute post-streptococcal complement deficiency

Chronic deficiencies in the complement pathway proteins are associated with an increased risk of meningococcal disease. Such deficiencies are caused by primary congenital immunodeficiency of a complement protein, properdin or mannose binding lectin, or are secondary to consumption of complement by systemic lupus erythematosus (SLE) or membranoproliferative glomerulonephritis (MPGN). Whatever the cause, the complement deficiency is always chronic. Here we report a case of meningococcal disease (MCD) in a child with a transient complement deficiency (CD), caused by post-streptococcal glomerulonephritis (PSGN).

Microbial genome dynamics in CNS pathogenesis

The balancing act between microbes and their host in commensal and disease states needs to be deciphered in order to fully treat and combat infectious diseases. The elucidation of microbial genome dynamics in each instance is therefore required. In this context, the major bacterial meningitis pathogens are Neisseria meningitidis, Haemophilus influenzae and Streptococcus pneumoniae. In prokaryotic CNS pathogenesis both the intact organism as well as its released components can elicit disease, often resulting in neurological sequelae, neurodegeneration or fatal outcome. The study of microbial virulence in CNS disease is expected to generate findings that yield new information on the general mechanisms of brain edema and excitatory neuronal disturbances due to meningitis, with significant potential for discoveries that can directly influence and inspire new strategies for prevention and treatment of this serious disease.

Human genetics of infectious diseases: a unified theory

Since the early 1950s, the dominant paradigm in the human genetics of infectious diseases postulates that rare monogenic immunodeficiencies confer vulnerability to multiple infectious diseases (one gene, multiple infections), whereas common infections are associated with the polygenic inheritance of multiple susceptibility genes (one infection, multiple genes). Recent studies, since 1996 in particular, have challenged this view. A newly recognised group of primary immunodeficiencies predisposing the individual to a principal or single type of infection is emerging. In parallel, several common infections have been shown to reflect the inheritance of one major susceptibility gene, at least in some populations. This novel causal relationship (one gene, one infection) blurs the distinction between patient-based Mendelian genetics and population-based complex genetics, and provides a unified conceptual frame for exploring the molecular genetic basis of infectious diseases in humans.