Behavioral and genetic investigations of low exploratory behavior in Il18r1(-/-) mice: we can't always blame it on the targeted gene.

The development of gene-targeting technologies has enabled research with immune system-related knockout mouse strains to advance our understanding of how cytokines and their receptors interact and influence a number of body systems, including the central nervous system (CNS). A critical issue when we are interpreting phenotypic data from these knockout strains is the potential role of genes other than the targeted one. Although many of the knockout strains have been made congenic on a C57BL/6 (B6) genetic background, there remains a certain amount of genetic material from the129 substrain that was used in the development of these strains. This genetic material could result in phenotypes incorrectly attributed to the targeted gene. We recently reported low-activity behavior in Il10(-/-) mice that was linked to this genetic material rather than the targeted gene itself. In the current study we confirm the generalizability of those earlier findings, by assessing behavior in Il18(-/-) and Il18r1(-/-) knockout mice. We identified low activity and high anxiety-like behaviors in Il18r1(-/-) mice, whereas Il18(-/-) mice displayed little anxiety-like behavior. Although Il18r1(-/-) mice are considered a congenic strain, we have identified substantial regions of 129P2-derived genetic material not only flanking the ablated Il18r1 on Chromosome 1, but also on Chromosomes 4, 5, 8, 10, and 14. Our studies suggest that residual 129-derived gene(s), rather than the targeted Il18r1 gene, is/are responsible for the low level of activity seen in the Il18r1(-/-) mice. Mapping studies are necessary to identify the gene or genes contributing to the low-activity phenotype.