Identification and characterization of thymus LIM protein: Targeted disruption reduces thymus cellularity

We have identified a novel LIM gene encoding the thymus LIM protein (TLP), expressed specifically in the thymus in a subset of cortical epithelial cel ls. TLP was identified as a gene product which is upregulated in a thymus i n which selection of T cells is occurring (Rag(-/-) OT-1) compared to its e xpression in a thymus in which selection is blocked at the CD4(+) CD8(+) st age of T-cell development (Rag(-/-) Tap(-/-) OT-1). TLP has an apparent mol ecular mass of 23 kDa and exists as two isomers (TLP-A and TLP-B), which ar e generated by alternative splicing of the message. The sequences of TLP-A and TLP-B are identical except for the C-terminal 19 or 20 amino acids. Bas ed on protein sequence alignment, TLP is most closely related to the cystei ne-rich proteins, a subclass of the family of LIM-only proteins. In both me dullary and cortical thymic epithelial cell lines transduced with TLP, the protein localizes to the cytoplasm but does not appear to be strongly assoc iated with actin. In immunohistochemical studies, TLP seems to be localized in a subset of epithelial cells in the cortex and is most abundant near th e corticomedullary junction. We generated mice with a targeted disruption o f the Tlp locus. In the absence of TLP, thymocyte development and thymus ar chitecture appear to be normal but thymocyte cellularity is reduced by appr oximately 30%, with a proportional reduction in each subpopulation.