The PedPAD study: boys predominate in the hypogammaglobulinaemia registry of the ESID online database.

Hypogammaglobulinaemias are the most common primary immunodeficiency diseases. This group of diseases is very heterogeneous, and little is known about these diseases in children. In the Pediatric Predominantly Antibody Deficiencies (PedPAD) study, we analysed data from the European Society for Immunodeficiencies (ESID) online database to gain more insight into the characteristics of children with hypogammaglobulinaemia; 46 centres in 18 different countries agreed to participate. Data from 2076 of the 3191 children who were registered at the time of data extraction with a diagnosis of hypogammaglobulinaemia (this excludes agammaglobulinaemia and defects in class-switch recombination) were available for analysis. The data set showed several limitations. Because of country-related differences in diagnostic criteria used for the classification of different types of primary hypogammaglobulinaemia, further analysis of the data was performed in the combined data set. The most striking observation is the strong majority of male patients in the group of children with primary hypogammaglobulinaemia (n = 1292, 63%). This male predominance was observed in each of the 18 countries involved. The boys were younger at diagnosis (mean age males 5·3 years; mean age females 5·8 years). Moreover, one or more complications were more frequently reported in boys (12%) compared to girls (5%). The male predominance suggests that patients with an undetected or unknown X-linked genetic cause are included in this group of children registered as primary hypogammaglobulinaemia.

Levels of somatic hypermutations in B cell receptors increase during childhood.

Somatic hypermutation (SHM) is an important step in antigen-driven B cell development creating B lymphocytes expressing high-affinity antibody receptors. It is known that the peripheral B lymphocyte compartments of healthy children and adults differ considerably. However, the development of SHM with age has not been studied in detail previously. Therefore, we used the immunoglobulin (Ig)κ-restriction enzyme hot-spot mutation assay (Igκ-REHMA) to gain an estimation of SHM levels in different age groups in order to relate this to the size of the memory B lymphocyte subpopulations. We show that the level of SHM increases rapidly during the first 2 years of life. This reflects the changes of the memory B cell subpopulations, but also changes in the SHM within memory B cell subsets, probably reflecting an increase of secondary memory B cell responses.

Paediatric reference values for the peripheral T cell compartment.

Immunophenotyping of blood lymphocyte subpopulations is an important tool in the diagnosis of immunological and haematological diseases. Paediatric age-matched reference values have been determined for the major lymphocyte populations, but reliable reference values for the more recently described T lymphocyte subpopulations, like different types of memory T lymphocytes, recent thymic emigrants, regulatory T cells and CXCR5(+) helper T lymphocytes, are not sufficiently available yet. We determined reference values for the absolute and relative sizes of T lymphocyte subpopulations in healthy children using the lysed whole blood method, which is most often used in diagnostic procedures. When the absolute numbers of some or all T lymphocyte subpopulations fall outside these reference ranges, this may indicate disease. The reference values show the course of T lymphocyte development in healthy children. Absolute T lymphocyte numbers increase 1.4-fold during the first months of life, and after 9-15 months, they decrease threefold to adult values; this is mainly caused by the expansion of recent thymic emigrants and naive cells. Helper and cytotoxic T lymphocytes show the same pattern. Regulatory T cells increase in the first 5 months of life and then gradually decrease to adult values, although the absolute numbers remain small. The relative number of CXCR5(+) cells within the CD4(+) CD45RO(+) T lymphocytes increases during the first 6 months of life and then remains more or less stable around 20%.

Age-matched reference values for B-lymphocyte subpopulations and CVID classifications in children.

Age-matched reference values are generally presented with 5th and 95th percentiles as 'normal' reference range. However, they are mostly determined in relatively small groups, which renders this presentation inaccurate. We determined reference values for B-lymphocyte subpopulations in healthy children with the statistical method of tolerance intervals that deals far better with the relatively small numbers tested, and compared these to the cut-off values used in the currently used EUROclass classification for common variable immunodeficiency disorders (CVID) in children. CVID is a heterogeneous group of primary immunodeficiency diseases characterized by low serum immunoglobulin levels and inadequate response to vaccination. Disease-modifying heterozygous amino acid substitutions in TACI are found in around ±10% of CVID patients. Interestingly, we found that age is the primary determinant of TACI-expression on B-lymphocytes, independent of switched memory B-lymphocyte numbers. Immunophenotyping of B-lymphocyte subpopulations is increasingly used to classify patients with CVID into subgroups with different clinical prognosis according to the composition of their B-lymphocyte compartment. These classifications were mainly developed with data obtained in adults. Because of the maturing paediatric immune system, they may not be equally applicable in children: our and other age-matched reference values show great changes in the composition of the B-lymphocyte compartment during development. Although the greatest changes in B-lymphocyte subpopulations occur below the age of 2 years, when the diagnosis of CVID cannot yet be made, it is likely that a classification developed in adults cannot be used to classify the prognosis of children.

Functionality of the pneumococcal antibody response in Down syndrome subjects.

We investigated the anti-polysaccharide antibody responses in subjects with Down syndrome (DS) because DS subjects show decreased peripheral B-lymphocyte numbers in all age groups, and a clinical picture of recurrent respiratory tract infections and increased incidence of autoimmune diseases which is reminiscent of common variable immunodeficiency disorders (CVID)-like disease. We determined titers and opsonophagocytosis in response to conjugated and unconjugated pneumococcal serotypes in 18 DS subjects aged 6-24 years. The results show adequate serotype-specific antibody titers in response to all conjugated and almost all unconjugated serotypes used. Opsonophagocytosis activity as measured against pneumococcal serotypes 9N, 19F and 23F was also found to be intact. We conclude that DS subjects do not have a clear defect in their anti-polysaccharide antibody response.