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41 p.-7 fig., ICAP-1 regulates β1 integrin activation and cell adhesion. Here we used ICAP-1-null mice to study ICAP-1 potential involvement during immune cell development and function. Integrin α4β1-dependent adhesion was comparable between ICAP-1-null and control thymocytes, but lack of ICAP-1 caused a defective single positive (SP) CD8+ cell generation, thus unveiling an ICAP-1 involvement in SP thymocyte development. ICAP-1 bears a nuclear localization signal and we found it displayed a strong nuclear distribution in thymocytes. Interestingly, there was a direct correlation between the lack of ICAP-1 and reduced levels in SP CD8+ thymocytes of Runx3, a transcription factor required for CD8+ thymocyte generation. In the spleen, ICAP-1 was found evenly distributed between cytoplasm and nuclear fractions, and ICAP-1-/- spleen T and B cells displayed upregulation of α4β1-mediated adhesion, indicating that ICAP-1 negatively controls their attachment. Furthermore, CD3+ - and CD19+ -selected spleen cells from ICAP-1-null mice showed reduced proliferation in response to T and B cell stimuli, respectively. Finally, loss of ICAP-1 caused a remarkable decrease in marginal zone B cell frequencies and a moderate increase in follicular B cells. Together, these data unravel an ICAP-1 involvement in the generation of SP CD8+ thymocytes and in the control of marginal zone B cell numbers., This work was supported by grants SAF2017-85146-R and PID2020-116291RB-I00 from the Ministerio de Ciencia e Innovación (MICINN) to J.T, PID2019-105623RB-I00 from MICINN to M.L.T,BFU2013-48828-P from MICINN to Y.R.C., ERC Synergy Grant (2018) to R.F., RTI2018-095497-B-I00 from MICINN to A.H, and RTI2018-093938-B-I100 from MICINN and (RD16/0011/0002, TERCEL) from Instituto de Salud Carlos III to AGZ.