T cell maturation, activation & differentiation (chapter 10) summary progenitor t cells from bone marrow enter thymus and rearrange tcr genes thymocytes rearrange tcr genes to become t cells small percentage rearrange to become t cells. Dn (double negative) cells lack cd4 and cd8 (early thymocytes) During development, double negative thymocytes develop (expression of either. Positive selection: eliminates t cells unable to recognize self mhc mhc. Negative selection: eliminate thymocytes bearing high affinity receptors for self. Mhc molecules (alone or self mhc plus self antigen) self tolerance. T cell activation initiated by interaction of tcr-cd3 complex with a peptide- Activation requires accessory molecules including co-receptors cd4/cd8. Intracellular signal transduction pathways activated by tcr engagement. T cells expressing cd4 recognize mhc-ii and function as th cells. T cells expressing cd8 recognize mhc-i and function as tc cells. Co-stimulatory signal induced by interaction between b7 on apc and cd28 on. Signal 2 th co-stimulatory signal provided by apc.