Gene targeting strategy for B-hCD19 mice. 
The coding sequence of human extracellular domain, mouse transmembrane and cytoplasmic domain was inserted into murine exon 2 in B-hCD19 mice.

Strain-specific CD19 expression analysis in B-hCD19 mice by flow cytometry. Spleens were collected from wild-type C57BL/6 mice (+/+), heterozygous B-hCD19 mice (H/+), and homozygous B-hCD19 mice (H/H), and analyzed by flow cytometry using an anti-CD19 antibody. Mouse CD19 was detectable in wild-type and heterozygous B-hCD19 mice (H/+). Human CD19 was exclusively detectable in heterozygous B-hCD19 (H/+) and homozygous B-hCD19 (H/H) mice.

Frequency of leukocyte subpopulations in spleen by flow cytometry. Splenocytes were isolated from wild-type C57BL/6 mice and homozygous B-hCD19 mice (n=3, 8-week-old). A. Flow cytometry analysis of the splenocytes was performed to assess the frequency of leukocyte subpopulations. B. Frequency of T cell subpopulations. Percentages of T cells, B cells, NK cells, dendritic cells, neutrophils, monocytes, macrophages, CD4+ T cells, CD8+ T cells and Tregs in B-hCD19 mice were similar to those in C57BL/6 mice, demonstrating that humanization of CD19 does not change the frequency or distribution of these cell types in spleen. The frequency of leukocyte subpopulations in bone marrow and blood of B-hCD19 mice were also comparable to wild-type C57BL/6 mice (Data not shown). Values are expressed as mean ± SEM. Significance was determined by two-way ANOVA test.  *P < 0.05, **P < 0.01, ***p < 0.0001. 
Spleen B cells were significantly reduced in homozygous null CD19 mice. (Engel P et al., 1995)