New insights into B cells as antigen presenting cells
Introduction
Professional antigen-presenting cells (APC) that constitutively express major histocompatibility complex class II molecules (MHCII) include dendritic cells (DC), monocyte/macrophages, B cells, thymic epithelia. Each of these cells interact with CD4+ T cells, with specialized antigen presentation functions related to the stage of CD4+ T cell development and differentiation. The importance of B cells as APC was highlighted by studies of B cell-specific MHCII conditional knockout (CD19cre-MHCIIfl/fl) mice, including in disease models [1, 2, 3]. Notably, several features of the MHCII antigen presentation pathway in B cells are distinctive and contribute to their uniqueness as APCs. These features include the expression of the B cell receptor (BCR) for antigen and regulated expression of HLA-DO.
Here we discuss recent progress in investigation of B cells as APC. We start with an update on the class II antigen presentation pathway in B cells. Turning to function, we highlight the roles of B cells as thymic APC, as APC for T follicular helper (TFH), as APC for CD4 memory T cells and as presenters of idiotypic BCR determinants. Along the way, we note recent examples that link B cell Ag-presentation to disease and unanswered questions.
Section snippets
The MHCII antigen presentation pathway in B cells
The current model of the MHCII antigen presentation pathways in B cells is summarized in Figure 1. Below, we focus on new findings related to this model. For comprehensive summaries of prior evidence supporting the model, the reader is referred to several reviews [4, 5, 6].
B cells as Ag presenting cells
Distinct roles of B cells as APC are shown schematically in Figure 2. Recent findings related to these specialized roles are discussed below.
Concluding remarks
More detailed pictures of the class II Ag presentation pathway in B cells and the functions of B cells as APC are emerging. The findings indicate that, in addition to unique features of B cells compared to other professional APC, there is appreciable heterogeneity among B cells. These variations appear to arise from differences in B cell subset, activation state, microenvironment and antigen type. Further elucidation of the mechanistic bases of these differences is a logical next step in the
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
•• of outstanding interest
Acknowledgements
This work was supported by grants from N.I.H. (R21AI095813) and Lucile Packard Foundation for Children’s Health to EDM.
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