TACI expression and APRIL binding on EBV-transformed cell lines. (A) The histograms show the mean fluorescence intensity (MFI) of stainings of EBV-transformed B-cell lines with a polyclonal antibody against TACI (left), a monoclonal antibody recognizing the TACI extracellular domain (clone 1A1; middle), and a Flag-tagged recombinant APRIL construct. ♦ indicates C104R homozygous (n = 2); ◇, C104R heterozygous (n = 12); , healthy donors (n = 5); and , CVID patients without TNFRSF13B mutation (n = 5). The thick gray line in each graph indicates the background staining of a previously described homozygous S144X cell line.¹⁰  Mean values and SDs are shown at the right side of the scatter diagram of each group. Testing for significance was performed with the Student t test. (B) TACI expression and APRIL binding on EBV-transformed cell lines with homozygous or compound heterozygous TNFRSF13B mutations, stained with a polyclonal antibody directed against the TACI extracellular domain (top panels), stained with a monoclonal antibody recognizing the TACI extracellular domain (middle panels), and stained with a Flag-tagged APRIL construct (bottom panels). Thick black lines indicate patients; thick dashed black lines, healthy donors; and gray filled histograms, patients with TNFRSF13B null mutation (S144X homozygous, negative control).