Transfer of antigen from human B cells to dendritic cells.

The cooperation of B lymphocytes with other antigen presenting cells (APCs) is often necessary in the efficient processing and presentation of antigen. Herein, we describe a mechanism by which B cells physically interact with dendritic cells (DCs) resulting in the transfer of B cell receptor (BCR)-enriched antigen to these APCs. Antigen transfer involves direct contact between the two cells followed by the capture of B cell derived membrane and intracellular components. Strikingly, DCs acquire greater amounts of antigen by transfer from B cells than by endocytosis of free antigen. Blocking scavenger receptor A, a DC surface receptor involved in membrane acquisition, abrogates these events. We propose that antigen transfer from B cells to DCs results in a more focused immunologic response due to the selective editing of Ag by the BCR.

Epstein-Barr virus promotes interferon-alpha production by plasmacytoid dendritic cells.

OBJECTIVE: Epstein-Barr virus (EBV) infection has been linked to systemic lupus erythematosus (SLE), as demonstrated by the presence of increased seroprevalence and elevated viral loads, but the mechanism of this linkage has not been elucidated. Increased interferon-alpha (IFNalpha) levels and signatures, which are associated with innate immune responses, have been found in patients with SLE. Plasmacytoid dendritic cells (PDCs) are innate immune cells that mediate viral immunity by producing large quantities of IFNalpha, but the role they play during infection with EBV remains unclear. To address this issue, we investigated the ability of EBV to promote IFNalpha production by PDCs in healthy subjects. METHODS: Human PDCs were sorted and cultured in the presence of EBV, EBV-encoded RNA, and EBV double-stranded DNA. IFNalpha production by PDCs was measured by enzyme-linked immunosorbent assay, with the activation of these cells measured by flow cytometry. RESULTS: We found that EBV DNA and RNA promoted IFNalpha production by human PDCs through engagement of Toll-like receptor 9 (TLR-9) and TLR-7, respectively, with the initial viral recognition by PDCs mediated by binding to class II major histocompatibility complex (MHC) molecules. CONCLUSION: These data demonstrate that class II MHC-specific engagement by virus, with subsequent viral nucleic acid recognition, mediates IFNalpha production by PDCs. Our results suggest that elevated levels of IFNalpha found in SLE patients may be a result of aberrantly controlled chronic viral infection.

Editing antigen presentation: antigen transfer between human B lymphocytes and macrophages mediated by class A scavenger receptors.

B lymphocytes can function independently as efficient APCs. However, our previous studies demonstrate that both dendritic cells and macrophages are necessary to propagate immune responses initiated by B cell APCs. This finding led us to identify a process in mice whereby Ag-specific B cells transfer Ag to other APCs. In this study, we report the ability and mechanism by which human B lymphocytes can transfer BCR-captured Ag to macrophages. The transfer of Ag involves direct contact between the two cells followed by the capture of B cell-derived membrane and/or intracellular components by the macrophage. These events are abrogated by blocking scavenger receptor A, a receptor involved in the exchange of membrane between APCs. Macrophages acquire greater amounts of Ag in the presence of specific B cells than in their absence. This mechanism allows B cells to amplify or edit the immune response to specific Ag by transferring BCR-captured Ag to other professional APCs, thereby increasing the frequency of its presentation. Ag transfer may perpetuate chronic autoimmune responses to specific self-proteins and help explain the efficacy of B cell-directed therapies in human disease.

Culture and analysis of circulating fibrocytes.

Fibrocytes circulate in the peripheral blood, produce collagen and other matrix proteins, and express cell surface markers indicative of a hematopoetic origin distinguishing them from fibroblasts. Circulating fibrocytes were first identified in 1994 in a model system of wound repair, and defined by their growth characteristics and unique surface phenotype. The methods currently employed for the isolation, growth, and characterization of peripheral blood fibrocytes rely on the entry of "fibroblast-like" cells into wound chambers, or the derivation of "fibroblast-like" cells from the buffy coat of peripheral blood obtained from different mammalian species. In this protocol, we culture fibrocytes from the mononuclear cells of peripheral blood and harvest the cultured cells for flow cytometry analysis.

The role of circulating fibrocytes in fibrosis.

Fibrocytes are cells that circulate in the peripheral blood and produce connective tissue proteins such as vimentin and collagens I and III. Fibrocytes are associated with skin lesions, pulmonary fibrosis, and tumors and they contribute to the remodeling response by secreting matrix metalloproteinases. Fibrocytes can further differentiate, and they are a likely source of the contractile myofibroblast that appears in many fibrotic lesions. There is evidence in the skin for a prominent role for fibrocytes in the development of hypertrophic scars and keloids. In asthma or in experimental models of pulmonary fibrosis, fibrocytes have been shown to infiltrate areas of inflammation and tissue damage. Fibrocytes constitute part of the stromal response to tumor invasion, and there is evidence that these cells may be a prognosticator of malignant potential. IL-1, TGF-beta, chemokines, and serum amyloid P modulate the appearance and function of fibrocytes. Fibrocytes themselves produce inflammatory cytokines, growth factors, and chemokines. The intercellular signals that modulate fibrocyte trafficking, proliferation, and differentiation are only partially defined, but a better understanding of these signals enable new therapies to prevent pathologic fibrosis or to improve the tissue repair response.

Circulating fibrocytes: collagen-secreting cells of the peripheral blood.

Since the original description of circulating fibrocytes in 1994, our knowledge of this unique cell population has grown steadily. While initially described in the context of wound repair, fibrocytes have since been found to participate in granuloma formation, antigen presentation, and various fibrosing disorders. Fibrocytes produce matrix proteins such as vimentin, collagens I and III, and they participate in the remodeling response by secreting matrix metalloproteinases. Fibrocytes also are a rich source of inflammatory cytokines, growth factors, and chemokines that provide important intercellular signals within the context of the local tissue environment. Moreover, fibrocytes express the immunological markers typical of an antigen-presenting cell, and they are fully functional for the presentation of antigen to naïve T cells. Fibrocytes can further differentiate, and they may represent a systemic source of the contractile myofibroblast that appears in many fibrotic lesions. Clinically, there is evidence that patients with hypertrophic scars such as keloids, and those affected by scleroderma and other fibrosing disorders have fibrocytes in their lesions. Recently, a new disease entity called nephrogenic fibrosing dermopathy (NFD) has been described, and the fibrocyte may play an important etiopathogenic role in disease development. Nephrogenic fibrosing dermopathy occurs in patients with renal insufficiency and leads to thickening and hardening of the skin, especially of the extremities. Ongoing research is focusing on the molecular signals that influence fibrocyte migration, proliferation, and function in the context of normal physiology and pathology.