Leukocyte-specificprotein 1 (LSP1) is a 52 kDa cytoplasmic F-actin binding phosphoprotein expressedin all human and murine leukocytes and endothelial cells. LSP1 is an important regulatorof actin cytoskeleton remodeling. We have previously shown that Lsp1-/- micehave an impaired CTL response after antigen exposure, with Lsp1-/- dendriticcells (DCs) failing to induce a strong CTL response in vivo. In order to studythe role of LSP1 in DCs to promote a CD4+ Tcell-mediated response, we first evaluated the capacity of Lsp1-/- DCsto activate CD4+ T cells. In vitro, bone marrow dendritic cells(BMDCs) were derived from Lsp1-/- mice with Flt3-L.BMDCs were pulsed with soluble or particulated Ovalbumin (OVAw or OVAb,respectively), then stimulated with CpG-ODN 1826 and finally cocultured withCD4+Tcells purified from OT-II mice, previously stained with e- Fluor 670proliferation dye. Three days later, a significantly lower percentage ofproliferating cells was observed with Lsp1-/- BMDCsincubated with OVAw (p< 0.0001) and OVAb (p< 0.0001), and CD25 expressionwith Lsp1-/-BMDCsincubated with OVAb (p< 0.005) compared to Lsp1+/+ BMDCs.In vivo, CD4+Tcells from OT-II mice were equally stained and injected into Lsp1-/- mice.Twenty-four hours later, mice were immunized with OVAw+CpG-ODN. Three days later,we observed a significantly lower frequency of proliferating CD4+ Tcell in draining lymph nodes (p< 0.05) in Lsp1-/- micecompared to Lsp1+/+ mice. These results suggest that LSP1deficiency affects CD4+ T cell activation,which could impair induction of effector responses in mice.