FUNCTIONAL-ANALYSIS OF PERIPHERAL-BLOOD B-CELLS IN PATIENTS WITH X-LINKED AGAMMAGLOBULINEMIA

X-linked agammaglobulinemia (XLA) is a primary immunodeficiency diseas e caused by mutations of Bruton tyrosine kinase (Btk); Btk plays an es sential role in the development of mature B cells. However, small numb ers of B cells (''leaky B cells'') are present in the peripheral blood of most XLA patients. In this study, we analyzed the function of thes e leaky B cells obtained from XLA patients. Enough numbers of B cells were available for analysis from five of mine XLA patients originally screened. Sequence analysis revealed missense mutations of Btk in four of the five XLA patients. No mutation was found in the coding region of Btk in one patient, Western blotting and/or flow cytometric analysi s failed to detect Btk protein in all five patients. B cells isolated from peripheral blood of these XLA patients were CD5(-), CD20(+), CD19 (+), and CD21(--). If stimulated with anti-CD40 and IL-4, XLA B cells proliferated normally and produced significant amounts of IgE. Anti-CD 40 stimulation of XLA B cells resulted in normal expression of CD23. I n addition, three of the five XLA patients studied were immunized with bacteriophage phi X174 and produced low but detectable levels of anti phage-specific Ab, Similarly, X-linked immunodeficiency mice, which ca rry a missense mutation in Btk, produced substantial amounts of antiph age Ab, These results indicate that CD40 signaling is intact in B cell s lacking demonstrable Btk, and that leaky B cells in XLA patients can proliferate, undergo isotype switching, and differentiate into specif ic Ab-producing cells.