Catherine Peronne

Cord blood B cells are mature in their capacity to switch to IgE-producing cells in response to interleukin-4 in vitro.

Neonatal B cells have been considered immature because of their impaired capacity to produce immunoglobulins in response to polyclonal activators in vitro. Here we demonstrate that cord blood mononuclear cells (MNC) produce normal levels of IgE in vitro when cultured in the presence of interleukin‐4 (IL‐4), indicating that the B cells are mature in their capacity to switch to IgE‐producing cells. However, in contrast to adult peripheral blood T cells, cord blood T cells failed to produce detectable levels of IL‐4 upon activation by phytohaemagglutinin (PHA) concanavalin A (Con A) or combinations of PHA and the phorbol ester TPA. Interferon‐gamma (IFN‐γ) production by cord blood T cells following activation by Con A or PHA was also strongly reduced. However, high levels of IFN‐γ, significantly higher than those produced by adult T cells, were synthesized in response to combinations of PHA and TPA, indicating that IFN‐γ production by cord blood T cells is not intrinsically defective. In contrast, cord blood T cells produced levels of IL‐2 that were significantly higher than those obtained by adult T cells tested in parallel. Collectively, our data indicate that the minimal levels of IgE production measured in cord blood (< 1 U/ml) are not due to immaturity of the cord blood B cells, but may be associated with the failure of cord blood T cells to produce detectable levels of IL‐4, which has been shown to be responsible for induction of IgE synthesis both in vitro and in vivo.

Cloning and characterization of the mouse homologue of the human dendritic cell maturation marker CD208/DC-LAMP

DC‐LAMP, a member of the lysosomal‐associated membrane protein (LAMP) family, is specifically expressed by human dendritic cells (DC) upon activation and therefore serves as marker of human DCmaturation. DC‐LAMP is detected first in activated human DC within MHC class II molecules‐containing compartments just before the translocation of MHC class II‐peptide complexes to the cell surface, suggesting a possible involvement in this process. The present study describes the cloning and characterization of mouse DC‐LAMP, whose predicted protein sequence is over 50% identical to the human counterpart. The mouse DC‐LAMP gene spans over 25 kb and shares syntenic chromosomal localization (16B2‐B4 and 3q26) and conserved organization with the human DC‐LAMP gene. Analysis of mouse DC‐LAMP mRNA and protein revealed the expression in lung peripheral cells, but also its unexpected absence from mouse lymphoid organs and from mouse DC activated either in vitro or in vivo. In conclusion, mouse DC‐LAMP is not a marker of mature mouse DC and this observation raises new questions regarding the role of human DC‐LAMP in human DC.

T Cells Can Induce Somatic Mutation in B Cell Receptor-Engaged BL2 Burkitt’s Lymphoma Cells Independently of CD40-CD40 Ligand Interactions

The B cell surface trigger(s) and the molecular mechanism(s) of somatic hypermutation remain unknown, partly because of the lack of amendable in vitro models. Recently, however, we reported that upon B cell receptor cross-linking and coculture with activated T cells, the Burkitt’s lymphoma cell line BL2 introduces mutations in its IgVH gene in vitro. We now confirm the relevance of our culture model by establishing that the entire spectrum of somatic mutations observed in vivo, including insertions and deletions, could be found in the DNA of BL2 cells. Additionally, we show that among four human B cell lines, only two with a centroblast-like phenotype can be induced to mutate. Triggering of somatic mutations in BL2 cells requires intimate T-B cell contacts and is independent of CD40-CD40-ligand (CD40L) interactions as shown by 1) the lack of effect of anti-CD40 and/or anti-CD40L blocking Abs on somatic mutation and 2) the ability of a CD40L-deficient T cell clone (isolated from an X-linked hyper-IgM syndrome patient) to induce somatic mutation in B cell receptor-engaged BL2 cells. Thus, our in vitro model reveals that T-B cell membrane interactions through surface molecules different from CD40-CD40L can trigger somatic hypermutation.

The capacity of interleukin-4 to induce in vitro IgE synthesis by B cells of patients with common variable immunodeficiency

Interleukin‐4 (IL‐4) has been shown to induce IgE synthesis by peripheral blood mononuclear cells (PBMC) of normal donors in vitro. However, induction of PBMC of patients with common variable immunodeficiency (CVI) with IL‐4 resulted in IgE production in only two out of eight cases tested. PBMC of the first patient that produced IgE in response to IL‐4 also secreted normal levels of IL‐4 upon activation. PBMC of the second patient secreted very low levels of IL‐4 in vitro which may account for the very low serum IgE levels in this patient. Of the other six patients who had very low serum IgE levels and whose PBMC failed to produce IgE in response to IL‐4 in vitro, five did not secrete IL‐4 upon in vitro activation. The capacity of the T cells to produce IL‐4 was intact in the sixth patient. Collectively our data indicate the PBMC of the majority of patients with CVI are defective since they failed to respond appropriately to IL‐4 and they failed to produce IL‐4, contributing to the view that CVI is a heterogeneous disorder in which a variety of T and B cell defects occur.