Leukocyte adhesion deficiency (LAD)-III is associated with homozygous stop codon mutations in Kindlin-3, the hematopoietic member of the Kindlin family of integrin coactivators. In addition, a subgroup of LAD-III patients has a homozygous splice junction mutation in and reduced expression of the Rap-1 guanine nucleotide exchange factor, CalDAG-GEFI (CDGI). In this study, we compared the adhesive properties of the leukocyte function-associated antigen-1 (LFA-1) and very late activation antigen-4 (VLA-4) integrins in both primary and activated leukocytes derived from these 2 LAD-III subgroups. Primary lymphocytes lacking both Kindlin-3 and CDGI lost all firm T-cell receptor-stimulated LFA-1 adhesiveness, in contrast to LAD-III lymphocytes deficient in Kindlin-3 alone. Effector T cells expanded from all tested LAD-III variants expressed normal CDGI, but lacked Kindlin-3. These Kindlin-3-null effector T cells exhibited total loss of inside-out IntroductionIntegrins constitute the major and largest family of cell adhesion receptors. 1 In hematopoietic cells, these heterodimers rapidly undergo dramatic allosteric conformational changes in response to various activation signals. 2 Talins are key integrin activators implicated in these processes in essentially all integrin-containing cell types. 3 Activated integrins are essential for platelet aggregation, firm leukocyte adhesiveness to vascular endothelium, and lymphocyte arrest on antigen-presenting cells. 4 Recent evidence suggests that Kindlins, a group of 3 structurally related adaptors, cooperate with talin in activating integrins in different cell types through binding to distinct motifs on the short tails of the integrin ␤ subunits. [5][6][7][8] In contrast to Kindlins-1 and -2, Kindlin-3 expression is restricted to the hematopoietic system. 7 Whereas loss of talin1 is embryonically lethal, deletion of Kindlin-3 is not; 7 yet, deletion of Kindlin-3 in mice results in severe defects in platelet and leukocyte integrin activation and in reduced lymphocyte counts, 7,9 osteoporosis, 7 and abnormal erythrocyte function, which is ultimately fatal. 10 Although a recent study has directly implicated murine Kindlin-3 in neutrophil and monocyte integrin adhesiveness to inflamed endothelium in vitro and in vivo, 9 the role of Kindlin-3 in inside-out (chemokine-mediated) and outside-in (ligand-induced) activation of lymphocyte integrins has remained unexplored.Leukocyte adhesion deficiency (LAD)-III is a rare autosomal recessive syndrome, which is manifested as a combined defect in ␤ 3 , ␤ 2 , and ␤ 1 integrin activation in platelets, neutrophils, and lymphocytes. 11 We previously reported 3 LAD-III cases from families of Turkish origin, associated with a homozygous splice junction mutation that results in defective expression of a key Rap-1-specific guanine nucleotide exchange factor (Rap-1/2 GEF), CalDAG-GEFI (CDGI), in platelets, neutrophils, and resting lymphocytes. 12 We recently showed that these patients suffer from an additional mutation, a homozygous nonsense stop ...

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Talin1 is a key integrin coactivator. We investigated the roles of this cytoskeletal adaptor and its target integrins in B- IntroductionProper circulation of B lymphocytes between primary and secondary lymphoid organs is critical for their differentiation and function in the establishment of normal humoral protection. [1][2][3] In adult mice, B-cell precursors rearrange their immunoglobulin (Ig) genes and differentiate into immature B cells in the bone marrow (BM). 4,5 After negative selection, these cells egress from BM niches, enter the circulation, and home to the spleen where they reach final maturation or die. 6 Immature spleen B cells can develop into either marginal zone (MZ) B cells or follicular B cells based on signaling through the B-cell receptor, Notch2, and the canonical nuclear factor-B pathway. 7 MZ B cells are a sessile population that reside at the border between the white and red pulps, and respond vigorously to blood-borne T cell-independent antigens. 8 Follicular (FO) B cells are generated from immature B cells entering the white pulp 3 and that differentiate into transitional 1 (T1) and transitional 2 (T2) cells based on their surface phenotype and functional characteristics. 9,10 The T1 Ͼ T2 Ͼ FO pathway is considered the main B-cell differentiation pathway in the spleen and is critical for the ability of B cells to respond to blood-borne T-dependent antigens. 11 A fraction of the mature follicular B lymphocytes leave the spleen and circulate in peripheral lymph nodes and mucosal-associated lymphoid tissues. Subsets of spleen and lymph node B cells can also home back to the BM, mainly after encountering antigen, and they differentiate into plasma or memory subsets. 12 Similar to T-cell circulation through various lymphoid organs, the entry of B subsets into the BM, lymph nodes, and the spleen involves their crossing of different types of endothelial barriers. 13 A key checkpoint in this emigration of both T and B cells is their ability to firmly attach to and protrude through the endothelial cell barrier lining these organs. 14 Whereas leukocyte intravasation (egress) into blood 15 is thought to take place independently of integrin-ligand interactions, B lymphocyte extravasation from the blood is thought to be mediated by distinct integrin-ligand interactions that generate shear-resistant adhesions. 14,16 Nevertheless, once in the extravascular tissues, interstitial motility of B lymphocytes, like other leukocytes, is thought to take place independently of integrins via chemokine cues. 17,18 Accumulating studies have established a key role for 3 B-cell integrins, VLA-4 (␣4␤ 1 ), ␣ 4 ␤ 7 , and LFA-1 (␣ L ␤ 2 ) in the trafficking of mature B cells to lymph nodes, the BM, 19 and peripheral sites of inflammation. 20 Although LFA-1 has a key role in B-cell migration to peripheral lymph nodes and Peyer patches, 21 it plays redundant roles in lymphocyte entry to the spleen and its white pulp. 2,21 Similarly, although the roles of ␣ 4 integrins in B-cell trafficking to mucosal tissues and the BM a...