Cdk5 regulates migrational and proliferative capacity of donor T cells after allo-HCT. Isolated Cdk5^+/+C (WT) T cells were stained with 2.5 μL of CFSE and Cdk5^−/−C (KO) T cells were stained with 5 μL of SNARF-1. Equal numbers of WT and KO T cells (3-5 × 10⁶) were coinjected into lethally irradiated B6D2F1 mice. Spleens and LNs were isolated from recipient mice at 24 and 48 hours after injection and cells were stained with CD4 or CD8 fluorescent antibodies and examined by flow cytometry. The percentage of CD4⁺ and CD8⁺ from WT (CFSE⁺) and KO (SNARF-1⁺) and percentage reduction of cells from KO mice were determined (A-B). In vivo T-cell proliferation was assessed in parallel experiments. T cells from Cdk5^+/+C or Cdk5^−/−C were labeled with CFSE and 5 × 10⁶ T cells were separately injected IV into lethally irradiated B6D2F1 mice. Seventy-two hours later, mice were sacrificed and the spleens were removed. Proliferating cells from WT or KO donors were identified based upon decreased staining for CFSE (C-D). Percentage reduction at 72 hours likely reflects effects of Cdk5 on both migration to SLOs and subsequent proliferation once present. N = 4 mice per group and represent 1 of at least 3 replicate experiments.