CD137L regulates gene expression in GC B cells. CD19⁺GL7⁺ GC B cells were sorted by flow cytometry from spleen and lymph nodes of WT and CD137L^−/− mice 9 days after immunization with NP-CG (left panel, GC1) or infection with influenza virus (middle panel, GC2). GC B cells of 4 mice per genotype were pooled for each experiment. mRNA was isolated and used for comparative expression analysis by genome-wide microarray. The heat map depicts transcript levels according to the indicated ²log ratio between expression in WT vs CD137^−/− GC B cells (see color scale). Green denotes lower expression, and red denotes greater expression in WT GC B cells compared with CD137^−/− GC B cells. The list of genes was selected for statistically significant differential expression (P < .01) and presence in both experiments. CD19⁺GL7⁻ non-GC B cells (right panel, nonGC) were sorted simultaneously and analyzed to confirm GC specificity of the listed genes.