The study by Jeong and colleagues (1) investigated P2X5-deficient (P2X5−/−) mice in a Listeria monocytogenes infection model, where they found a pronounced susceptibility of the P2X5−/− mice to L. monocytogenes infection. This was accompanied by a reduced release of mature IL-1β by P2X5−/− bone marrow–derived macrophages.
The P2rx5 gene lies on mouse chromosome 11 (Chr11:73160421-73172685 bp). After a personal communication, the authors confirmed that the mouse used in their study was the P2X5−/− mouse generated by Lexicon Genetics (B6;129S5-P2rx5tm1Lex/Ieg; MGI:5430769). The embryonic stem cells used to generate these mice originated from the 129S5/SvEvBrd strain. Because the obtained P2X5−/− mice were backcrossed onto the C57BL/6 background, it is possible that the P2X5−/− mice used by Jeong et al. harbor passenger mutations in the flanking regions of the targeted P2rx5 gene that are still of 129S5/SvEvBrd origin. Even if new knockout mice are backcrossed for 10 generations, the probability that a 1 cM region flanking the targeted gene is retained from the donor embryonic stem cells is, in theory, still 91% (2, 3). Indeed, P2rx5 is located only 2 Mb upstream of the highly polymorphic Nlrp1 gene locus, consisting of Nlrp1a (Chr11:71092236-71144704 bp), Nlrp1b (Chr11:71153102-71230733 bp), and the pseudogene Nlrp1c. Past studies revealed that macrophages from 129 related substrains express functional Nlrp1b but lack mRNA expression of the other two isoforms. In contrast, C57BL/6 mice express Nlrp1a but lack functional Nlrp1b expression (4–6). The presence of an Nlrp1 passenger mutation has been demonstrated already by Gerlic and colleagues for the Srepb1a-deficient (Srepb1a−/−) mouse (B6.129P2-Srebf1Gt(XC354)Byg) (7). Interestingly, IL-1β maturation was also diminished in bone marrow–derived macrophages from Srepb1a−/− mice (8). Gerlic et al. further state in their letter (7) that caution is advised when using mice knocked out for any gene in the vicinity of the Nlrp1 locus. Given the fact that P2rx5, at a distance of ∼2 Mb, is much closer to the Nlrp1 cluster than the Srebf1 gene (Chr11:60199089-60222581) targeted in Srepb1a−/− mouse, it is conceivable to speculate that the Nlrp1 passenger mutation is also present in the P2X5−/− mice used by Jeong and colleagues. Because the authors state that P2X5−/− mice exhibit defective inflammasome activation, it seems reasonable to evaluate the possibility that an Nlrp1 passenger mutation could, at least in part, be responsible for the tremendous phenotype of the P2X5−/− mice in the L. monocytogenes infection model.