Unraveling the role of invariant Natural Killer T-cells in the immune response to Streptococcus pneumoniae.
Streptococcus pneumoniae is a leading cause of life threatening infections like meningitis, pneumonia and septicaemia and the main pathogen involved in otitis and bronchitis. Pneumococcal capsular polysaccharides are the most important virulence factor of S. pneumoniae. Anti-polysaccharide antibodies have shown to be crucial for opsonisation and effective phagocytosis of S. pneumoniae, thereby providing protection against early phase disease. Patients with polysaccharide antibody deficiency (PsAD) suffer from recurrent upper and lower respiratory tract infections due to a deficient antibody response to capsular polysaccharides. This defect can present isolated (Specific Antibody Deficiency or SAD) or in the context of a broader immunodeficiency. Correct diagnosis is crucial to initiate appropriate treatment like immunoglobulin replacement therapy or prophylactic antibiotics, preventing irreversible organ damage (hearing loss, bronchiectasis). PsAD is diagnosed by measuring anti-polysaccharide IgG prior to and after Pneumococcal Polysaccharide Vaccine (PPV), but this method faces many challenges. The gold standard is a laborious and expensive WHO standardized ELISA. Guidelines to define a normal response remain controversial. Furthermore, standard vaccination of children with protein-conjugated pneumococcal polysaccharides (PCV) induces a T dependent antibody response, and is likely to influence the magnitude and nature of the antibody response to PPV. Finally, reports on hyporesponsiveness to subsequent vaccinations with polysaccharide vaccines raise concerns on the potential harmful effect of immunizing immunocompromised patients with PPV.
Our first aim was to facilitate and improve diagnosis of PsAD. In Chapter 1, we demonstrated the need for alternatives methods to detect PsAD by showing that previous vaccination with PCV influences the response to PCV-PPV shared serotypes but also to non-PCV serotypes. Allohemagglutinins (AHA), antibodies to A and B polysaccharides on erythrocytes, are mentioned in many immunology reviews and textbooks as an alternative method to detect PsAD. Only a few dated studies described normal values of AHA and research on the diagnostic value of AHA to detect SAD was lacking. We conducted a retrospective study in 180 patients, and found low sensitivity and specificity of AHA to detect PsAD and no correlation of low AHA with low PPV response (Chapter 2). More importantly, low AHA did not associate with clinical signs of PsAD like recurrent lower respiratory tract infections and bronchiectasis. In Chapter 3, we aimed to investigate Typhim Vi vaccine as an alternative for PPV to diagnose PsAD. Typhim Vi vaccine contains Vi capsular polysaccharides from Salmonella typhi, a new or ‘neo-antigen’ for most European individuals, facilitating interpretation of the response. Hundred healthy volunteers were vaccinated with Typhim Vi and Pneumovax vaccine. We established normal values for post-vaccination IgG and fold increase of antibody titres pre- to post-vaccination after Typhim Vi vaccination. We confirmed that AAAAI criteria for PsAD cannot be applied to bead-based assay results for PPV response. Instead we propose serotype-specific 5th percentile cut-off values for post-vaccination IgG and fold increase post PPV measured by bead-based assay. Finally, we also assessed AHA in these 100 healthy subject and demonstrated a large spread of AHA, with fifth percentile cut-offs much lower than the currently used thresholds, again questioning the clinical value of low AHA. A follow-up study in 100 patients with suspected humoral immunodeficiency is currently recruiting participants to validate our findings in a patient population.
Despite the importance of antibodies to capsular polysaccharides in the protection against pneumococcal disease, the mechanisms inducing a class switched IgG response to polysaccharides are still poorly understood. A number of ‘second signals’ required for an IgG response to polysaccharides have been identified and Primary Immunodeficiencies (PIDs) affecting one of the ‘second signal’ pathways are associated with PsAD (e.g. MyD88 and IRAK4 deficiency causing abnormal TLR signalling). However, in most patients with isolated PsAD (thus SAD) and in many patients with PID, a cause has not yet been found. The second aim of this thesis, was to investigate the role of invariant Natural Killer T (iNKT) cells in the pneumococcal polysaccharide antibody response in humans. iNKT cells are a specialized subset of T cells that use their restricted set of T cell receptors to recognize self and foreign lipids presented by CD1d on B cells and antigen presenting cells. Whilst the involvement of iNKT cells in the innate and adaptive response to S. pneumoniae in mice has clearly been demonstrated, the evidence in humans is scarce. We described the association of absent iNKT cells and pneumococcal infections in two siblings from consanguineous parents. Pneumococcal polysaccharide response was completely abolished. Based on the clinical presentation, a very rare calcium release activated calcium channelopathy due to STIM1 mutation was suspected and confirmed by genetic and functional testing (Chapter 4). This report was the first to describe prolonged survival in STIM1 deficiency, and auto-inflammatory manifestations other than auto-immune cytopenia namely severe colitis and psoriasis. In the last chapter (Chapter 5), we describe an unusual presentation of autosomal dominant hyper IgE syndrome with normal IgE, normal polysaccharide antibody response and normal iNKT cell numbers. Low iNKT cells in patients with PsAD and increased susceptibility of patients with low iNKT to S. pneumoniae infections hint towards a role of iNKT cells in the generation of polysaccharide antibody response. However, further studies are needed to elucidate the exact role of iNKT cells in T independent responses and to reveal the pathways involved. Multiple second signals are required for a fully functional polysaccharide response. The future identification of underlying genetic defects in patients with PsAD by next generation sequencing will hopefully shed a light on redundant and non-redundant signals for the polysaccharide antibody response.