Semaphorin 4A is dynamically regulated during thymocyte development in mice

• Semaphorin 4A protein and mRNA levels are downregulated as CD4−CD8− cells mature.
• Semaphorin 4A protein is localized to the thymic medulla.
• Semaphorin 4A mRNA and protein are present on CD4−CD8−CD44+CD25− (DN1) thymocytes.
• SEMA4A protein in DN1 cells is restricted to the CD117− subset of DN1 cells.
• Sema4A modulation in thymocytes suggests a complex regulation of migratory cues.

Semaphorins are important regulators of peripheral T and B-cell mediated immune responses in mice and humans. Modulatory roles of semaphorins in T cell development are also being characterized. We carefully analyzed the gene expression and protein levels of semaphorins 4A, 4D, and 7A at various developmental stages of T cell maturation in the thymus of C57BL/6 mice. Sema7a was expressed at very low levels, while Sema4d was abundant at all developmental stages of mouse thymocytes. We found the most interesting pattern of gene regulation and protein localization for semaphorin 4A. Both semaphorin 4A mRNA and protein were clearly detected on the earliest progenitors and were downregulated through thymic development. SEMA4A protein also showed a distinct cortico-medullary pattern of localization. Our findings contribute to an understanding of the complex roles played by semaphorins in the network of spatially and temporally regulated cues underpinning T cell development in the thymus.