As illustrated in Figure 1A-I (tonsil) and Figure 1A-II (RA synovium), CNA.42 colocalizes with an Ab specific for the FDC-restricted CD21 long isoform (CD21L) in the FDC reticula; and additionally, labels extra-reticular FDCs that are CNA.42+/CD21L−. Here, PPP3CA is linked to rheumatoid arthritis.