The inducible costimulator (ICOS) is the newest member of the CD28/CD152 receptor family involved in regulating T cell activation. We constructed a soluble-Ig fusion protein of the extracellular domain of human ICOS and used it as a probe to characterize expression patterns of the ICOS ligand (ICOSL). ICOSIg did not bind to CD80- or CD86-transfected Chinese hamster ovary cell lines, demonstrating that ICOSL is distinct from those ligands identified for CD28/CD152. ICOSIg showed selective binding to monocytic and B cell lines, whereas binding was undetectable on unstimulated monocytes and peripheral blood T and B cells. Expression of ICOSL was induced on monocytes after integrin-dependent plastic adhesion. Pretreatment of monocytes with mAb to the β2-integrin subunit CD18 decreased adhesion and abolished ICOSL up-regulation but had no effect on CD80/86 (CD152 ligand (CD152L)) expression. Both ICOSL and CD152L were up-regulated on monocytes by IFN-γ but by distinct signaling pathways. Unlike CD152L expression, ICOSL expression did not change when monocytes were differentiated into dendritic cells (DCs) or after DCs were induced to mature by LPS, TNF-α, or CD40 ligation. Addition of ICOSIg to allogeneic MLRs between DCs and T cells reduced T cell proliferative responses but did so less efficiently than CTLA4Ig (CD152Ig) did. Similarly, ICOSIg also blocked Ag-specific T cell proliferation to tetanus toxoid. Thus, ICOSL, like CD80/86, is expressed on activated monocytes and dendritic cells but is regulated differently and delivers distinct signals to T cells that can be specifically inhibited by ICOSIg.

Monoclonal antibodies against the T-cell activation molecule 4-1BB have been effective in the treatment of established mouse tumors. To create a vaccine that stimulates the immune system similarly to the efficacious monoclonal anti-4-1BB antibody, 1D8, we constructed a vector encoding cell-bound single-chain Fv fragments from 1D8. We transfected the vector into cells from the K1735 melanoma, selected because of its low immunogenicity and very low expression of major histocompatibility complex class I. The transfected cells induced a strong type 1 T-helper cell response, for which CD4+ but not CD8+ T lymphocytes were necessary and that involved natural killer cells. Vaccinated mice rejected established wild-type K1735 tumors growing as subcutaneous nodules or in the lung. An analogous approach may be effective against micrometastases in human patients, including tumors whose expression of major histocompatibility complex class I is very low.

CD37 is a lineage-specific B-cell antigen that to date has been neglected as an attractive therapeutic target. To exploit this, novel CD37-specific small modular immunopharmaceuticals ( IntroductionImmunotherapy using monoclonal antibodies (MAbs) is emerging as a safe and selective method for the treatment of cancer. 1 The role of monoclonal antibodies in B-cell malignancies has expanded since the introduction of rituximab (Rituxan) targeted against the CD20 antigen on the B-cell surface in 1997. Numerous studies have confirmed the efficacy of rituximab as a single agent and in combination therapy in low-grade non-Hodgkin lymphoma (NHL), 2-6 mantle-cell lymphoma, 7-11 diffuse large-cell lymphoma, 12,13 and Burkitt leukemia/lymphoma. 14 However, only a subset of patients respond to therapy and the majority of those eventually relapse after rituximab treatment. Therefore, identification of new therapeutic targets on B cells that are potentially more effective than CD20 represents a novel strategy for therapy of B-cell malignancies.The CD37 antigen is one potential target that has not been adequately evaluated. CD37 is a heavily glycosylated 40-to 52-kDa glycoprotein and a member of the tetraspan transmembrane family of proteins. 15,16 CD37 is expressed strongly on the surface of B cells and transformed mature B-cell leukemia and lymphoma cells [17][18][19][20]22,23,25,26 but is either absent or minimally expressed on normal T cells. 21 The CD37 antigen is expressed on monocytes and granulocytes at very low density and is absent on natural killer (NK) cells, platelets, and erythrocytes. 15,22 During B-cell development, CD37 is expressed in cells progressing from pre-B to peripheral mature B-cell stages and is absent on terminal differentiation to plasma cells. 23 Although the precise function of CD37 remains unknown, it has been found to form complexes with CD53, CD81, CD82, and class II glycoprotein on B-cell surface that may represent an ion channel or a transporter. 24 CD37 has modest internalization and shedding in transformed B cells expressing the antigen. 25,26 It is highly expressed in endosomes and exosomes in B lymphocytes, reflecting possible involvement in intracellular trafficking and antigen presentation. 15 Targeted inactivation of CD37 in mice revealed no changes in the development of lymphoid organs but a reduced IgG1 level in the sera and an alteration of response to T-cell-dependent antigens, indicating a possible role of CD37 in T cell-B cell interaction. 27 Given the relative B-cell selectivity, CD37 thus represents a valuable therapeutic target for malignancies derived from peripheral mature B cells, such as B-cell chronic lymphocytic leukemia (CLL), hairy-cell leukemia (HCL), and B-cell NHL. 25,26 In particular, CLL may be a good target of CD37-based immunotherapy, because the expression of CD37 is relatively high, even compared with CD20, in this type of leukemia. 17 Efforts to target CD37 clinically have been limited. One reported preclinical trial performed in the late 1980s examined the ...

We recently found that human CD83, a marker of mature dendritic cells, is an adhesion receptor that binds to resting monocytes and a subset of activated CD8+ T cells. We injected CD83-Ig into mice transplanted with the immunogenic P815 mastocytoma and showed that it significantly enhanced the rate of tumor growth and inhibited the development of cytotoxic T cells. In contrast, mice immunized with CD83-transfected K1735 cells, a poorly immunogenic melanoma, could prevent the outgrowth of wild-type K1735 cells. Studies performed in vitro with human PBL showed that coimmobilized CD83-Ig and anti-CD3 enhanced T cell proliferation and increased the proportion of CD8+ T cells. CD83-transfected B-lymphoblastoid T51 cells stimulated T cell proliferation more effectively than untransfected T51 cells in MLR cultures and increased the generation of cytolytic T cells. We conclude that CD83 is a functionally important receptor that can regulate the development of cellular immunity by interacting with its ligand(s).

To help determine CD83 function, a cDNA encoding a soluble protein containing the CD83 extracellular domain was fused with a mutated human IgG1 constant region (CD83Ig) and expressed by stable transfection of Chinese hamster ovary cells. Purified CD83Ig bound to peripheral blood monocytes and a subset of activated CD3+CD8+ lymphocytes but did not bind to FcR. Monocytes that had adhered to plastic lost their ability to bind to CD83Ig after 90 min of in vitro incubation. CD83Ig bound to two of five T cell lines tested, HPB-ALL and Jurkat. The binding to HPB-ALL cells significantly increased when they were grown at a low pH (pH 6.5), whereas binding to Jurkat cells increased after apoptosis was induced with anti-Fas mAb. B cell and monocytic lines did not bind CD83Ig and neither did CD56+ NK cells or granulocytes. Full-length CD83 expressed by a transfected carcinoma line mediated CD83-dependent adhesion to HPB-ALL cells. CD83Ig immunoprecipitated and immunoblotted a 72-kDa protein from HPB-ALL cells. Binding of CD83Ig to HPB-ALL cells was eliminated by neuraminidase treatment of the cells. We conclude that CD83 is an adhesion receptor with a counterreceptor expressed on monocytes and a subset of activated or stressed T lymphocytes, and that interaction between CD83 and its counterreceptor is dependent upon the state of glycosylation of a 72-kDa counterreceptor by sialic acid residues. In view of the selectivity of the expression of CD83 and its ligand, we postulate that the interaction between the two plays an important role in the induction and regulation of immune responses.