WHIM mutation CXCR4^E343K attenuates CXCL12-induced receptor down-regulation but to a lesser degree than CXCR4^R334X. Surface expression was measured with CXCR4-specific mAb 12G5 for the cell types indicated at the top of each panel after incubation in the presence or absence of the stimuli indicated on the x-axis. (A) Transiently transfected K562 cells. **P < .01 and ***P < .001 for the comparison between CXCR4^WT (WT) and CXCR4^E343K (E343K) or CXCR4^R334X (R334X), respectively. Data are summarized as the means ± SEM from 4 experiments. (B) PBMCs. Data are pooled from multiple independent experiments with cells with the indicated CXCR4 genotypes (number of individuals studied for each genotype). P1-P3 were studied twice and P4 once; 1 CXCR4^R334X patient was studied twice and the other 2 once, and each HD was studied once. One or 2 HDs were studied as controls in each of the 5 experiments involving WHIM patients, and the rest were studied in 8 additional independent experiments. Data are the means ± SEM of all values in each category. *P < .05; **P < .01; and ***P < .001 for the comparison between HDs and CXCR4^R334X (R334X) or CXCR4^E343K (E343K) patients, respectively. (C) B cells. After incubation with stimuli in duplicates, PBMCs were gated on CD19⁺ cells and analyzed for CXCR4 expression. Data are from 1 experiment that included WHIM patients P1-P4, 1 CXCR4^R334X patient, and 1 HD, and 2 additional independent experiments for the other 4 HDs, and are the means ± SEM of all values in each category. **P < .01 and ***P < .001 for the comparison between HDs and CXCR4^R334X (R334X) or CXCR4^E343K (E343K) patients, respectively. (D) Normal homologous desensitization of WHIM variants CXCR4^R334X (R334X) and CXCR4^E343K (E343K) expressed in K562 cells. Data were pooled from 5 independent experiments done in triplicate with the indicated variants and are summarized as the means for each condition.