The expression of the Flk2/Flt3 molecule (CD135), the receptor for Flt3 ligand (FL), was investigated in the human thymus. Results showed that there is a high level of expression of CD135 by thymocyte populations, especially by intrathymic T-cell precursor populations. As these results suggested a role for FL in the regulation of thymic T-cell precursor differentiation and/or proliferation, we used an in vitro model of thymic stromal cell cultures in order to delineate the activity of FL on human CD7(high)CD3-CD4-CD8- triple negative intrathymic T-cell precursors. Results showed that FL, either alone or in combination with stem cell factor (SCF) induced the proliferation of CD7(high) precursors, but to a lower extent than interleukin-7 (IL-7) or IL-7 + SCF, used as positive controls. In the presence of FL + SCF, CD7(high) cells developed mainly towards a CD11b+ phenotype whereas IL-7 + SCF preferentially induce a CD3+TcRgammadelta+CD8+ phenotype. Taken together, these data suggest that FL may play a role in inducing the proliferation of CD7(high) human intrathymic T-cell precursors, but also in the induction of a myeloid differentiation pathway within the human thymus.

When bacterial cells come in contact, antagonism mediated by the delivery of toxins frequently ensues. The potential for such encounters to have long-term beneficial consequences in recipient cells has not been investigated. Here we examined the effects of intoxication by DddA, a cytosine deaminase delivered via the type VI secretion system (T6SS) of Burkholderia cenocepacia. Despite its killing potential, we observed that several bacterial species resist DddA and instead accumulate mutations installed by the toxin, indicating that even in the absence of killing, interbacterial toxins can have profound consequences on target populations. Investigation of additional toxins from the deaminase superfamily revealed that mutagenic activity is a common feature of these proteins, including a representative we show targets single-stranded DNA and displays a markedly divergent structure. Our findings suggest that a surprising consequence of antagonistic interactions between bacteria could be the promotion of adaptation via the action of directly mutagenic toxins.

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