Chih-Chao Chang

Molecular pathogenesis of B cell malignancy: the role of BCL-6.

Human malignancies displaying a mature B cell phenotype include non-Hodgkin lymphoma, (NHL), chronic lymphocytic leukemia (CLL), and multiple myeloma (MM). Analogous to most cancer types, the pathogenesis of these malignancies represents a multistep process involving the progressive and clonal accumulation of multiple genetic lesions affecting proto-oncogenes and tumor suppressor genes. However, several important features distinguish the mechanism and type of genetic alterations associated with NHL, CLL, and MM (Table 1).

Soluble Ig-Like Transcript 3 Inhibits Tumor Allograft Rejection in Humanized SCID Mice and T Cell Responses in Cancer Patients

Attempts to enhance patients’ immune responses to malignancies have been largely unsuccessful. We now describe an immune-escape mechanism mediated by the inhibitory receptor Ig-like transcript 3 (ILT3) that may be responsible for such failures. Using a humanized SCID mouse model, we demonstrate that soluble and membrane ILT3 induce CD8+ T suppressor cells and prevent rejection of allogeneic tumor transplants. Furthermore, we found that patients with melanoma, and carcinomas of the colon, rectum, and pancreas produce the soluble ILT3 protein, which induces the differentiation of CD8+ T suppressor cells and impairs T cell responses in MLC. These responses are restored by anti-ILT3 mAb or by depletion of soluble ILT3 from the serum. Immunohistochemical staining of biopsies from the tumors and metastatic lymph nodes suggests that CD68+ tumor-associated macrophages represent the major source of soluble ILT3. Alternative splicing, resulting in the loss of the ILT3 transmembrane domain, may contribute to the release of ILT3 in the circulation. These data suggest that ILT3 depletion or blockade is crucial to the success of immunotherapy in cancer. In contrast, the inhibitory activity of soluble ILT3 on T cell alloreactivity in vitro and in vivo suggests the potential usefulness of rILT3 for immunosuppressive treatment of allograft recipients or patients with autoimmune diseases.

Ig-Like Transcript 3 Regulates Expression of Proinflammatory Cytokines and Migration of Activated T Cells

Ig-like transcript 3 (ILT3), an inhibitory receptor expressed by APC is involved in functional shaping of T cell responses toward a tolerant state. We have previously demonstrated that membrane (m) and soluble (s) ILT3 induce allogeneic tolerance to human islet cells in humanized NOD/SCID mice. Recombinant sILT3 induces the differentiation of CD8+ T suppressor cells both in vivo and in vitro. To better understand the molecular mechanisms by which ILT3 suppresses immune responses, we have generated ILT3 knockdown (ILT3KD) dendritic cells (DC) and analyzed the phenotypic and functional characteristics of these cells. In this study, we report that silencing of ILT3 expression in DC (ILT3KD DC) increases TLR responsiveness to their specific ligands as reflected in increased synthesis and secretion of proinflammatory cytokines such as IL-1α, IL-1β, and IL-6 and type I IFN. ILT3KD-DC also secretes more CXCL10 and CXCL11 chemokines in response to TLR ligation, thus accelerating T cell migration in diffusion chamber experiments. ILT3KD-DC elicit increased T cell proliferation and synthesis of proinflammatory cytokines IFN-γ and IL-17A both in MLC and in culture with autologous DC pulsed with CMV protein. ILT3 signaling results in inhibition of NF-κB and, to a lesser extent, MAPK p38 pathways in DC. Our results suggest that ILT3 plays a critical role in the in control of inflammation.

BCL-6 AND THE MOLECULAR PATHOGENESIS OF B-CELL LYMPHOMA

The results presented identify the first genetic lesion associated with DLCL, the most clinically relevant form of NHL. Although no proof yet exists of a role for these lesions in DLCL pathogenesis, the feature of the BCL-6 gene product, its specific pattern of expression in B cells, and the clustering of lesions disrupting its regulatory domain strongly suggest that deregulation of BCL-6 expression may contribute to DLCL development. A more precise definition of the role of BCL-6 in normal and neoplastic B-cell development is the goal of ongoing study of transgenic mice engineered either to express BCL-6 under heterologous promoters or lacking BCL-6 function due to targeted deletions. In addition to contributing to the understanding of DLCL pathogenesis, the identification of BCL-6 lesions may have relevant clinical implications. DLCL represent a heterogeneous group of neoplasms which are treated homogeneously despite the fact that only 50% of patients experience long-term disease-free survival (Schneider et al. 1990). The fact that BCL-6 rearrangements identify biologically and clinically distinct subsets of DLCL suggests that these lesions may be useful as markers in selection of differential therapeutic strategies based on different risk groups. Furthermore, the BCL-6 rearrangements can be used to identify and monitor the malignant clone with sensitive PCR-based techniques. Since clinical remission has been observed in a significant fraction of DLCL cases, these markers may serve as critical tools for sensitive monitoring of minimal residual disease and early diagnosis of relapse (Gribben et al. 1993).

Alterations of the BCL-6 gene in diffuse large-cell lymphoma.

The molecular analysis of specific chromosomal translocations has improved our understanding of the pathogenesis of various non-Hodgkin lymphoma (NHL) subtypes including follicular lymphoma (FL), Burkitt’s lymphoma (BL) and mantle-cell lymphoma, which are characterized by chromosomal translocations causing the deregulated expression of the BCL-2, C-MYC, and BCL-1, respectively (for review see Korsmeyer 1992, Dalla-Favera 1993, and Gaidano and Dalla-Favera 1992). However, relatively little is known about the molecular pathogenesis of diffuse large cell lymphoma (DLCL), the most frequent and most lethal human lymphoma (Magrath 1990). DLCL accounts for ~40% of initial NHL diagnoses and is often the final stage of progression of FL (Magrath 1990). Several molecular alterations have been detected at variable frequency in these tumors, but none has been specifically or consistently associated with the disease (Ladanyi et al. 1991, Offit et al. 1989a).

Advances in the Understanding of the Molecular Pathogenesis of Aggressive B Cell Lymphomas

Non-Hodgkin lymphoma (NHL) include neoplasms originating from lymphoid cells and characterized by a high degree of biological and clinical heterogeneity (for review see Magrath, 1990). Most NHL derive from the B-cell lineage, in particular from mature B-cells characterized by rearranged immunoglobulin (Ig) heavy and light chain genes and by the expression of cell surface Ig and B-cell associated markers. The wide clinico-pathological heterogeneity of NHL correlates with distinct genetic lesions, particularly chromosomal translocations, associated with its pathogenesis (Table 1; Gaidano and Dalla-Favera, 1993). Among low-grade NHL, “mantle zone” lymphoma are associated in 50% of cases with the t(11;14) translocation involving the juxtaposition of the IgH locus to the BCL-1/PRAD-1/cyclin D1 gene coding for a protein involved in the control of cell cycle progression (Tsujimoto et al., 1984; Motokura et al., 1991; Raffeid et al., 1991). In follicular-type NHL(FL), the t(14;18) translocation juxtaposes the IgH locus to BCL-2 (Bakhshi et al., 1985; Tsujimoto et al., 1984; Cleary et al., 1985), a gene coding for a protein that prevents programmed cell death or apoptosis (Korsmeyer, 1992). After years of indolent course, a significant fraction of FL undergoes histologic transformation and clinical progression into Diffuse Large Cell Lymphoma (DLCL), an event which is associated with loss or mutations of the p53 tumor suppressor gene (Lo Coco et al., 1993). “De novo” DLCL are associated with rearrangements and deregulation of the BCL-6 gene, which codes for a zinc-finger transcription factor (Ye et al., 1993a, 1993b; Kerckaert et al., 1993). In Burkitt Lymphoma (BL), the t(8;14), t(8;22), and t(2;8) chromosomal translocations lead to the deregulated expression of the c-Myc proto-oncogene by juxtaposition to one of the Ig loci (Dalla-Favera et al., 1982; Taub et al., 1982; Dalla-Favera et aI., 1983; Dalla-Favera, 1991). A sizable fraction (35%) ofBL are also associated with loss or mutations of the p53 gene (Gaidano et aI., 1991).