Composition of precursor B-cell compartment in bone marrow from patients with X-linked agammaglobulinemia compared with healthy children.

X-linked agammaglobulinemia (XLA) is characterized by a severe B-cell deficiency, resulting from a differentiation arrest in the bone marrow (BM). Because XLA is clinically and immunologically heterogeneous, we investigated whether the B-cell differentiation arrest in BM of XLA patients is heterogeneous as well. First, we analyzed BM samples from 19 healthy children by flow cytometry. This resulted in a normal B-cell differentiation model with eight consecutive stages. Subsequently, we analyzed BM samples from nine XLA patients. Eight patients had amino acid substitutions in the Bruton's tyrosine kinase (BTK) domain or premature stop codons, resulting in the absence of functional BTK proteins. In seven of these eight patients a major differentiation arrest was observed at the transition between cytoplasmic Ig(mu-) pre-B-I cells and cytoplasmic Ig(mu+) pre-B-II cells, consistent with a role for BTK in pre-B-cell receptor signaling. However, one patient exhibited a very early arrest at the transition between pro-B cells and pre-B-I cells, which could not be explained by a different nature of the BTK mutation. We conclude that the absence of functional BTK proteins generally leads to an almost complete arrest of B-cell development at the pre-B-I to pre-B-II transition. The ninth XLA patient had a splice site mutation associated with the presence of low levels of wild-type BTK mRNA. His BM showed an almost normal composition of the precursor B-cell compartment, suggesting that low levels of BTK can rescue the pre-B-cell receptor signaling defect, but do not lead to sufficient numbers of mature B lymphocytes in the peripheral blood.