Chronic lymphocytic leukemia (CLL) is a malignant disease of small mature lymphocytes. Previous studies have shown that CLL B lymphocytes express relatively large amounts of CD74 mRNA relative to normal B cells. In the present study, we analyzed the molecular mechanism regulated by CD74 in B-CLL cells. The results presented here show that activation of cell-surface CD74, expressed at high levels from an early stage of the disease by its natural ligand, macrophage migration-inhibition factor (MIF), initiates a signaling cascade that contributes to tumor progression. This pathway induces NF-B activation, resulting in the secretion of IL-8 which, in turn, promotes cell survival. Inhibition of this pathway leads to decreased cell survival. These findings could form the basis of unique therapeutic strategies aimed at blocking the CD74-induced, IL-8-dependent survival pathway.apoptosis ͉ invariant chain B cell chronic lymphocytic leukemia (B-CLL) is characterized by the progressive accumulation of CD5 ϩ B lymphocytes in peripheral blood, lymphoid organs, and bone marrow (1). The hallmark of the disease is decreased apoptosis, resulting in accumulation of these malignant cells. Previous studies have shown that chronic lymphocytic leukemia (CLL) lymphocytes express relatively large amounts of mRNA for CD74, which is the cell-surface form of invariant chain (Ii), as compared with normal B cells (2, 3). CD74 is a nonpolymorphic type II integral membrane protein, which was originally thought to function mainly as an MHC class II chaperone (4). However, CD74 recently was found to play an additional role as an accessory-signaling molecule. In macrophages, CD74 demonstrates high-affinity binding to the proinflammatory cytokine, macrophage migration-inhibitory factor (MIF). MIF binds to the extracellular domain of CD74; this complex is required for MIF-mediated MAPK activation and cell proliferation (5). Moreover, the bacterium Helicobacter pylori was shown to bind to CD74 on gastric epithelial cells and to stimulate IL-8 production (6).In a previous study, we showed that CD74 is involved directly in shaping the B cell repertoire (7, 8) by a pathway leading to the activation of transcription mediated by the NF-B p65/RelA homodimer and its coactivator, TAFII105 (9). NF-B activation is mediated by the cytosolic region of CD74 (CD74-ICD), which translocates to the cell nucleus (10). This signal is terminated by degradation of the active CD74-ICD fragment (11,12). Moreover, we demonstrated recently that CD74 stimulation with an agonistic CD74 antibody leads to NF-B activation, enabling entry of the stimulated B cells into the S phase, an increase in DNA synthesis, cell division, and augmented expression of members of the Bcl-2 protein family. Thus, these findings indicate that surface CD74 functions as a survival receptor (13).In the present study, we sought to determine whether CD74 functions as a survival receptor in B-CLL cells. Our results show that MIF-induced CD74 activation initiates a signaling cascade that results in secr...

The mechanism by which immature B cells are sequestered from encountering foreign antigens present in lymph nodes or sites of inflammation, before their final maturation in the spleen, has not been elucidated. We show here that immature B cells fail to home to the lymph nodes. These cells can actively exclude themselves from antigen-enriched sites by downregulating their integrin-mediated adhesion to the extracellular matrix protein, fibronectin. This inhibition is mediated by interferon γ secretion. Perturbation of interferon γ activity in vivo leads to the homing of immature B cells to the lymph nodes. This is the first example of autocrine regulation of immune cell migration to sites of foreign antigen presentation.

Immature B cells differentiate in the spleen into mature B cells, a process that is essential for their participation in the immune response. Previously, we showed that the MHC class II chaperone, invariant chain (Ii), controls this differentiation to the mature stage. Ii cytosolic domain-induced B cell maturation involves activation of transcription mediated by the NF-kappaB p65/RelA homodimer and requires the B cell enriched coactivator, TAF(II)105. In this study we show that the cytosolic region of Ii is cleaved within the plane of the membrane to generate a cytosolic fragment, which is essential for NF-kappaB activation and B cell differentiation. Our results suggest that Ii functions as a membrane-bound inactive inducer of NF-kappaB transcription that is activated by intramembrane proteolytic cleavage.

Early stages of B cell development occur in the bone marrow, resulting in formation of immature B cells. These immature cells migrate to the spleen where they differentiate into mature (B2 or marginal zone (MZ)) cells. This final maturation step is crucial for B cells to become responsive to Ags and to participate in the immune response. Id2 is a helix-loop-helix protein that lacks a DNA-binding region; and therefore, inhibits basic helix-loop-helix functions in a dominant negative manner. In this study, we show that Id2 expression is down-regulated during differentiation of immature B cells into mature B2 and MZ B cells. The high levels of Id2 expressed in the immature B cells result in inhibition of E2A binding activity to an E2 box site. Moreover, mice lacking Id2 show an elevation in the proportion of mature B2 cells in the spleen, while the MZ population in these mice is almost absent. Thus, Id2 acts as a regulator of the differentiation of immature B cells occurring in the spleen, it negatively controls differentiation into mature B2 cells while allowing the commitment to MZ B cells. In the absence of Id2 control, the unregulated differentiation is directed toward the mature B2 population.

Most mature follicular B cells circulate within the periphery in a quiescent state, without actively contributing to an acute immune response. Lasting B-cell persistence in the periphery is dependent on survival signals that are transduced by cell surface receptors. We recently demonstrated that cell surface CD74 controls mature B-cell survival. Stimulation of cell surface CD74 leads to NF-B activation, which enables entry of the stimulated B cells into the S phase, induction of DNA synthesis, and cell division, and augments the expression of survival genes. In the present study, we investigated CD74 target genes to determine the identities of the molecules whose expression is modulated by CD74, thereby regulating B-cell survival. We report that CD74 activates the p65 member of the NF-B family, which in turn up-regulates the expression of p53-related TAp63 proteins. IntroductionIn healthy individuals, the pool of peripheral lymphocytes is constant in size. The control of lymphoid homeostasis is the result of a very fine balance between lymphocyte production, survival, and proliferation. Survival factors have been shown to play a critical role in maintaining lymphocyte homeostasis.Recently, we demonstrated that CD74 (invariant chain, Ii) controls mature B-cell survival. CD74 is a nonpolymorphic type II integral membrane protein, containing a short N-terminal cytoplasmic tail of 28 amino acids (aa's), followed by a single 24-aa transmembrane region, and an approximately 150-aa lumenal domain. The CD74 chain was originally thought to function mainly as an MHC class II chaperone, promoting the exit of MHC class II molecules from the endoplasmic reticulum (ER), directing them to endocytic compartments, preventing peptide binding within the ER, and contributing to peptide editing in the MHC class II compartment. 1 However, CD74 was recently found to play an additional role as an accessory signaling molecule. A small proportion of CD74 is modified by the addition of chondroitin sulfate (CD74-CS), and this form of CD74 is expressed on the surface of antigen-presenting cells together with CD44. 2,3 In macrophages, CD74 was recently reported to demonstrate high-affinity binding to the proinflammatory cytokine, macrophage migration-inhibitory factor (MIF). MIF binds to the extracellular domain of CD74, and this interaction is required for MIF-mediated cell proliferation. 4 In a previous series of studies, we showed that CD74 is directly involved in shaping the B-cell repertoire 5 through a pathway leading to the activation of transcription mediated by the NF-B p65/RelA homodimer and its coactivator, TAFII105. 6 NF-B activation is mediated by the cytosolic region of CD74 (CD74-ICD), which is liberated from the membrane. 7 We demonstrated that following the removal of the CD74 lumenal domain, an intramembranal cleavage event at amino acid 42 occurs, resulting in the release of the CD74 cytosolic fragment (CD74-ICD; aa's 1-42). CD74-ICD, which has no NLS motif, then translocates to the nucleus and activates NF-B. 8 Thus, fol...