Ontogeny of B cells and pathogenesis of humoral immunodeficiencies

Low expression of the interleukin (IL)-4 receptor alpha chain and reduced signalling via the IL-4 receptor complex in human neonatal B cells

Diminished neonatal antibody responses following infection or immunization may stem in part from intrinsic characteristics of neonatal B cells. In this study, we used B-cell subset sorting combined with gene expression assays to investigate major differences in the expression of host genes in neonatal and adult naïve B cells. We discovered significantly reduced expression of the interleukin (IL)-4 receptor alpha chain and reduced IL-4-induced signalling in neonatal B cells. Neonatal naïve B cells were susceptible to more rapid and more profound levels of apoptosis when cultured in vitro. They also exhibited a limited response to IL-4 treatment compared with adult cells. The expression level of the IL-13 receptor alpha 1 chain, a key component of the IL-13 receptor/IL-4 type II receptor, and the response to IL-13 treatment for protection against apoptosis in neonatal B cells were similar to those of the adult B cells. These studies suggest a possible mechanism underlying the limited magnitude and durability of neonatal antibody responses.

Expression and Activity of β-Defensins and LL-37 in the Developing Human Lung

Immaturity of innate immunity contributes to the increased susceptibility of human neonates to infection. The lung is a major portal of entry for potential pathogens in the neonate, and human beta-defensins (HBDs) and LL-37 participate in pulmonary innate immunity. We hypothesized that these antimicrobial factors would be developmentally regulated, expressed by neonatal pulmonary tissues, and participate in neonatal innate immunity. We found HBD-2 to be the predominant beta-defensin in human neonatal lung. HBD-2 mRNA expression was developmentally regulated, induced by the proinflammatory factor IL-1beta, and decreased by dexamethasone. Additionally, HBD-2 abundance in neonatal tracheal aspirates increased as a function of gestational age. HBD-1 had a lower level of expression compared with HBD-2 and was induced by dexamethasone. HBD-3 and LL-37 messages were not detected in airway epithelial cultures. Additionally, each antimicrobial peptide exhibited a unique spectrum of antimicrobial activity and salt sensitivity against bacteria commonly causing sepsis in the neonate. Lower levels of HBD-2 may be one factor contributing to the increased susceptibility of premature infants to pulmonary infections.

Phenotypic characteristics of thymic B lymphocytes in myasthenia gravis

We have found prominent secretion of immunoglobulin and anti-acetylcholine antibodies by thymic lymphocytes (TL) of myasthenics despite a relative paucity of B (surface IgM+) cells in such TL. To determine whether there was a surface IgM- B cell in the TL which could manifest such responses, we compared the frequency of cells expressing the B cell-specific phenotypic marker CD20 (B1+), surface IgM (SIgM+), surface IgG (SIgG+), and surface IgD (SIgD+) in TL and autologous blood mononuclear cells in 36 myasthenic patients. B1+ cells were significantly more frequent than SIgM+ cells in TL (3.2 +/- 0.6 vs 0.6 +/- 0.2). In double-labeling studies, less than 25% of the B1+ cells coexpressed SIgM. Only 0.3% of the TL were SIgD+. In contrast, the frequencies of B1+ and SIgM+ cells in autologous blood were not significantly different (10.7 +/- 1.3 vs 8.2 +/- 0.8%). About 75% of blood B1+ cells co-expressed SIgM. These findings suggest that mast B cells in these TL have undergone isotope switching during prior in vivo differentiation and could manifest the observed humoral responses.

Humoral immunodeficiency: a review

Humoral immunodeficiency is a heterogeneous group of rare disorders. The availability of IVGG has meant significant improvement for many patients, but infections continue to occur even on Ig replacement therapy. This may be partially due to associated T cell defects, the inability of current therapy to provide antibody effectively to mucosal surfaces, or the difficulty of providing antibody of sufficiently high titer, affinity, or specificity during infection. A combination of maintenance Ig replacement coupled with increased amounts of Ig and appropriate antibiotics during specific infections remain the mainstays of current therapy.