TWEAK expression by PBMC. PBMC from two healthy volunteers were sorted by FACS based on their expression of CD22, yielding CD22+ and CD22- populations of greater than 94% purity based on post-sort analysis (A). Isolated cells were then analysed for TWEAK mRNA expression relative to that of GAPDH, by real-time RT-PCR (B). Data shown are means of triplicate reactions ± SD. Differences in relative expression of TWEAK mRNA between CD22+ and CD22- populations were tested by Student's t-test (**P < 0.001).