Lipid-antigen presentation by CD1d(+) B cells is essential for the maintenance of invariant natural killer T cells

Updates on B-cell immunotherapies for systemic lupus erythematosus and Sjogren's syndrome

PURPOSE OF REVIEW

Last year was marked by important clinical and mechanistic studies that improved our understanding of B-cell immunotherapy for systemic lupus erythematosus (SLE) and Sjogren's syndrome. Here, we will highlight the most relevant studies published in the last 18 months.

RECENT FINDINGS

The highlight of the year was the approval of belimumab on the basis of two major trials. On the flip side, the disappointing results of rituximab in lupus nephritis provided a clinical and mechanistic counterpoint in SLE. Still, major limitations in the LUpus Nephritis Assessment with Rituximab (LUNAR) trial, positive subset analysis and new open studies and registries continue to provide hope for and major insights into the use of B-cell depletion. In Sjogren's syndrome, the role of B-cell depletion has been further investigated, both for glandular and extraglandular manifestations of the disease with mixed results in a disease in which outcomes are notoriously hard to measure.

SUMMARY

The approval of anti-B cell activating factor therapy and an increasing body of open studies with rituximab as well as subset studies and secondary analysis of the Efficacy and Safety of Rituximab in Moderately-to-Severely Active Systemic Lupus Erythematosus (EXPLORER) and LUNAR trials provide hope for B-cell immunotherapy and significant insight into its mechanisms of action and utilization in a selected subset of patients. Ongoing clinical trials of other B-cell targeting agents are eagerly anticipated.

Revisiting the B-cell compartment in mouse and humans: more than one B-cell subset exists in the marginal zone and beyond

The immunological roles of B-cells are being revealed as increasingly complex by functions that are largely beyond their commitment to differentiate into plasma cells and produce antibodies, the key molecular protagonists of innate immunity, and also by their compartmentalisation, a more recently acknowledged property of this immune cell category. For decades, B-cells have been recognised by their expression of an immunoglobulin that serves the function of an antigen receptor, which mediates intracellular signalling assisted by companion molecules. As such, B-cells were considered simple in their functioning compared to the other major type of immune cell, the T-lymphocytes, which comprise conventional T-lymphocyte subsets with seminal roles in homeostasis and pathology, and non-conventional T-lymphocyte subsets for which increasing knowledge is accumulating. Since the discovery that the B-cell family included two distinct categories — the non-conventional, or extrafollicular, B1 cells, that have mainly been characterised in the mouse; and the conventional, or lymph node type, B2 cells — plus the detailed description of the main B-cell regulator, FcγRIIb, and the function of CD40⁺ antigen presenting cells as committed/memory B-cells, progress in B-cell physiology has been slower than in other areas of immunology. Cellular and molecular tools have enabled the revival of innate immunity by allowing almost all aspects of cellular immunology to be re-visited. As such, B-cells were found to express “Pathogen Recognition Receptors” such as TLRs, and use them in concert with B-cell signalling during innate and adaptive immunity. An era of B-cell phenotypic and functional analysis thus began that encompassed the study of B-cell microanatomy principally in the lymph nodes, spleen and mucosae. The novel discovery of the differential localisation of B-cells with distinct phenotypes and functions revealed the compartmentalisation of B-cells. This review thus aims to describe novel findings regarding the B-cell compartments found in the mouse as a model organism, and in human physiology and pathology. It must be emphasised that some differences are noticeable between the mouse and human systems, thus increasing the complexity of B-cell compartmentalisation. Special attention will be given to the (lymph node and spleen) marginal zones, which represent major crossroads for B-cell types and functions and a challenge for understanding better the role of B-cell specificities in innate and adaptive immunology.

The role of B lymphocytes in the progression from autoimmunity to autoimmune disease

Autoimmunity, defined as the presence of autoreactive T and/or B lymphocytes in the periphery, is a frequent and probably even physiological condition. It is mainly caused by the fact that the central tolerance mechanisms, which are responsible for counter-selection of autoreactive lymphocytes, are not perfect and thus a limited number of these autoreactive cells can mature and enter the periphery. Nonetheless, autoreactive cells do not lead automatically to autoimmune disease as evidenced by a multitude of experimental and human data sets. Interestingly, the progression from autoimmunity to autoimmune disease is not only determined by the degree of central tolerance leakage and thus the amount of autoreactive lymphocytes in the periphery, but also by peripheral mechanism of activation and control of the autoreactive cells. In this review, we discuss the contribution of peripheral B lymphocytes in this process, ranging from activation of T cells and epitope spreading to control of the autoimmune process by regulatory mechanisms. We also discuss the parallels with the role of B cells in the induction and control of alloimmunity in the context of organ transplantation, as more precise knowledge of the pathogenic antigens and time of initiation of the immune response in allo- versus auto-immunity allows better dissection of the exact role of B cells. Since peripheral mechanisms may be easier to modulate than central tolerance, a more thorough understanding of the role of peripheral B cells in the progression from autoimmunity to autoimmune disease may open new avenues for treatment and prevention of autoimmune disorders.

Natural killer T cell based Immunotherapy

Natural killer T (NKT) cells play an important immunoregulatory role and are thought to bridge the innate and adaptive immune responses. Following activation through cognate interactions with lipid antigen presented in the context of CD1d molecules, NKT cells rapidly produce a plethora of cytokines and can also mediate cytotoxicity. Due to their potent effector functions, extensive research has been performed to increase our understanding on how to effectively modulate these cells. In fact, NKT cell agonists have been used as vaccine adjuvants to enhance antigen specific T and B cell responses to infections and malignancy. In this review, we will focus on recent advances in NKT cell-based vaccination strategies. Given the role that NKT cells play in autoimmune disease, infectious diseases, cancer, transplant immunology and dermatology, it is important to understand how to effectively guide their effector functions in order to develop novel immunotherapeutic strategies.