Posterior kidney tissue samples were obtained from rainbow trout naturally infected with PKD. Samples were classified according to their kidney swelling grade, as detailed in the Methods. All control fish had a kidney swelling grade of 0 (Control) and samples were named Grade 1 (G1), Grade 1–2 (G1-2), Grade 2 (G2) and Grade 3 (G3). Transcriptional levels of BAFF (A), APRIL (B) and BALM (C) ligands, as well as the BAFF-R (A), BCMA (B) and TACI (C) receptors were evaluated by real time PCR. Results are shown as the gene expression relative to the expression of an endogenous control (EF-1α) (mean ± SD) (Control, n = 10; G1, n = 4, G1-2, n = 7; G2, n = 11, G3, n = 9). Statistical differences between control and infected groups were analyzed with a 2-tailed Student´s t test where * P<0.05, ** P<0.01 and *** P < 0.005. A linear regression is also included (dotted line) to show the correlation between the expression of specific genes and the course of the infection. The Pearson product-moment correlation coefficients (r) are also given relative to kidney swelling grade (indicated in the plots).