In contrast to T cells, activation of the BCR in blood B cells was not associated with changes in RhoH levels. These data suggest that RhoH function might be regulated by lysosomal degradation of RhoH protein following TCR complex but not BCR activation. This newly discovered regulatory pathway of RhoH expression might limit TCR signaling and subsequent T-cell activation upon Ag contact. RhoH (also known as

TTF) is a member of the Rho (ras homologous) GTPase subfamily of the Ras (rat sarcoma) superfamily of small GTP-binding proteins 1. RhoH mRNA expression was reported to be restricted to hematopoietic cells 1. Protein expression data are not available, click here except for one recent report, which demonstrated increased RhoH protein SAHA HDAC expression in GM-CSF-stimulated neutrophils 2. Rho GTPases are important intracellular

signaling molecules regulating the organization of the cytoskeleton, cell polarity, activation, proliferation, and survival (for review: 3). They usually cycle between an active, GTP-bound, and an inactive, GDP-bound, state. In contrast, RhoH has no measurable intrinsic GTPase activity and resides always in the active form 4. As a consequence, regulation of RhoH function appears to be only possible at the expression level, e.g. by modulating RhoH transcription 4 and/or alternative splicing 5, or by modifying its subcellular localization. Mice lacking RhoH have been independently generated by two research groups 6, 7. The phenotype of these mice revealed that RhoH is an important regulator of T-cell activation since deficiency of RhoH results in reduced T-cell differentiation and proliferation, and consequently in reduced numbers of T cells in the thymus, lymph nodes, and spleen 6, 7. Although the exact molecular mechanisms remain to be determined, Gu Y et al. suggested that RhoH recruits Zap70, a crucial tyrosine kinase in TCR signaling, to the immunological synapse 7. In contrast, Dorn T et al. proposed that RhoH regulates TCR signaling downstream of Zap70 6. In contrast to T cells,

the functional role of RhoH in primary B cells remains unknown. It is possible, however, that RhoH might Protirelin play a role in the pathogenesis of B-cell lymphomas since dysregulated RhoH expression has been reported in a number of B-cell malignancies 1, 8. T cells play central roles in all adaptive immune responses against pathogens. Since RhoH activity was shown to be crucial for T-cell activation 6, 7, it is important to study its regulation. We hypothesized that besides transcription 4 and alternative splicing 5, additional mechanisms might play a role that contribute to the regulation of RhoH expression and function. In this manuscript, we report RhoH protein expression levels in different blood cells and a new pathway of regulating RhoH protein expression in T cells, based on lysosomal degradation of the protein.