HSCs lose CD86 as they give rise to progenitors lacking lymphopoietic potential. (A) Multiparameter flow cytometry was used to resolve HSC-rich LSK (CD150⁺ CD48⁻), as well as LMPP (Flt3^Hi, CD150⁻ Rag1GFP⁻ LSK), ELP (Flt3^Hi, CD150⁻ Rag1GFP⁺), CLP (Lin⁻, Flt3⁺, IL-7Rα⁺, ckit⁺), CLP2 (Ly6C⁻, NK1.1⁻, IgM⁻, B220⁺, Flt3⁺, IL-7Rα⁺), ProB (Ly6C⁻, NK1.1⁻, IgM⁻, B220⁺, CD43⁺, CD19⁺), PreB (Ly6C⁻, NK1.1⁻, IgM⁻, B220⁺, CD43⁻, CD19⁺), and immature B (Ly6C⁻, NK1.1⁻, IgM⁺, B220⁺, CD19⁺) cells. (B) Down-regulation of CD86 corresponds to decreased myeloid potential and then absence of competence for lymphopoiesis. The lymphoid primed Lin⁻ Flt3^Hi Sca-1⁺ c-Kit^Hi subset (LMPP) was subdivided according to CD86 and placed in defined, serum-free, stromal cell-free cultures for 14-16 days. Flow cytometric analysis was then performed (top panels), and the results were calculated as yields per input progenitor (histograms). (C) A slightly less primitive Lin⁻ c-Kit⁺ fraction lacking Sca-1⁺ and c-Kit^Hi progenitors was sorted according to CD86 and cultured for 10 days. Statistically significant differences are indicated by asterisks (**P ≤ .01). Similar results were obtained in 3 independent experiments.