Human memory B cells: Memory B cells of a special kind (2008)

Memory B cells are generated during the course of T cell-dependent immune responses in histological structures called germinal centres. These cells have undergone somatic hypermutation of their rearranged Ig V region genes and many of them have also performed Ig class switch recombination. About 10 years ago, the expression of CD27 was proposed as a surface marker for human memory B cells.1, 2 However, there is now evidence that additional subsets of memory B cells exist.3 One of these subsets is defined by the expression of FCRL4, an inhibitory receptor that was previously known as IRTA1 or FcRH4.4, 5 Two studies published in a recent issue of the Journal of Experimental Medicine shed more light on these peculiar B cells. The study by Ehrhardt et al.6 provides a detailed account of the particular gene expression pattern of these cells compared to FCRL4− memory B cells, thereby further defining FCRL4+ cells as a distinct B-cell population. On the other hand, Moir et al.7 report the unexpected finding that FCRL4+ B cells are present at increased frequencies in the peripheral blood of human immunodeficiency virus (HIV) viraemic patients and have an ‘exhausted’ phenotype. In healthy individuals, FCRL4+ B cells are largely restricted to mucosal tissues and mesenteric lymph nodes, and are very rare in peripheral blood, lymph nodes and the spleen.4, 5Most FCRL4+ B cells do not express CD27, and ∼90% of them have undergone class switching predominantly to IgG, whereas a smaller proportion express IgA.4, 5 FCRL4+ B cells express high levels of CD20 and intermediate levels of CD21, but lack the expression of markers that identify germinal centre B cells and plasma cells.4, 5 Similar to classical memory B cells, most of the FCRL4+ B cells also express high levels of the CD80 and CD86 costimulatory molecules, have a propensity to quickly differentiate into antibody-secreting cells upon in vitro stimulation with mimics of T-cell help and carry somatically mutated Ig V genes (Figure 1).4, 5 These features collectively strongly suggest that FCRL4+ B cells represent memory cells. The study by Ehrhardt et al.6 now reveals multiple differences in terms of gene expression between FCRL4+ and FCRL4− memory B cells. These differences involve transcription factors, cell cycle regulators, signalling molecules and surface proteins. For example, FCRL4+ B cells display increased levels of the adhesion molecule and complement receptor CD11c, and about 10-fold higher levels of the Runx2 transcription factor but about 10-fold lower levels of Runx1 than FCRL4− memory B cells. As little is known about the genes regulated by Runx1 and Runx2 in mature B cells, it will be important to determine the role of these factors in the generation and function of the distinct memory B cells. Overall, this study defines multiple differences in gene expression between FCRL4+ and FCRL4− memory B cells and supports the concept that these B-cell subsets have different functions in immune responses. The FCRL4+ B cells detected by Moir et al.7 in the peripheral blood of HIV+ viraemic individuals share many phenotypic features with the tissue-based FCRL4+ memory B cells. Thus, this might indicate that in the disease situation of viraemic HIV-positive patients, FCRL4+ B cells exit mucosal tissues and circulate through the blood. However, the situation is likely not that simple, as there are also important differences between the FCRL4+ B cells in healthy individuals and in these viraemic HIV+ individuals. These differences include a low or largely absent expression of CD80 and CD86 on FCRL4+ B cells in HIV+ individuals, whereas these molecules are expressed at high levels in FCRL4+ B cells from healthy individuals (Figure 1). Moreover, many tissue-based memory B cells in healthy persons are Ki-67+ and in the G1 phase of the cell cycle, whereas FCRL4+ B cells in the setting of HIV viraemia appear largely Ki-67-negative, and hence are quiescient non-cycling cells. Importantly, there are also functional differences. FCRL4+ B cells isolated from tonsils of HIV-negative persons are induced to proliferate and differentiate into plasmablasts upon stimulation by CD40 ligand and interleukins, but FCRL4-expressing B cells from HIV+ persons do not proliferate strongly upon such stimulation and are also impaired in their ability to differentiate into antibody-secreting cells in response to polyclonal stimuli (Figure 1). Although FCRL4+ memory B cells in healthy humans show the same mutation load in their Ig V region genes as classic memory B cells, Ig V genes of FCRL4+ B cells from HIV viraemic patients are also less mutated. These characteristics of FCRL4+ B cells indicate that the cells may be exhausted, as proposed by Moir et al.7 ‘Exhaustion’ is a well-known phenomenon from T-cell immunology and describes a differentiation stage with expression of multiple inhibitory receptors, reduced expression of activating molecules and impaired effector functions. Importantly, as anti-HIV specificities were enriched among the FCRL4+ B cells, the impaired function of these cells may contribute to the poor humoral response against HIV.7 Although the two studies gave important and exciting novel insights into FCRL4+ B cells in humans, many questions remain to be resolved. What is the relationship between FCRL4+ memory B cells and the mostly FCRL4-negative CD27+ memory B cells? Can they develop from the same germinal centre B-cell clones? Is one population the precursor of the other, or are they generated in distinct immune responses? Or can FCRL4+ B cells even develop outside the germinal centres? Interestingly, FCRL4+ B cells show high transcript levels of activation-induced cytidine deaminase,6 the enzyme essential for somatic hypermutation and class switching. However, as these cells have a similar Ig V gene mutation load as CD27+ memory B cells, one can assume that they need to undergo a similar vigorous and extensive clonal expansion to acquire these mutations in repeated rounds of proliferation, mutation and selection. Such a strong proliferation of B cells, however, is not seen outside the germinal centres, favouring the idea that FCRL4+ B cells are generated in germinal centres. Phenotypic and functional characteristics of FCRL4-positive and -negative memory B cells. Shown are markers that are particularly prominent on the respective cells. If a marker is not shown, this may reflect that it is not expressed or expressed at reduced level in the cells. However, as not all markers were analysed for each of the three B-cell subsets, some molecules are not shown simply due to the lack of information about their expression in the respective B cells. FCRL4+ B cells in viraemic HIV patients express multiple inhibitory receptors, including CD22, CD72, CD85j, CD85k and LAIR-1.7 For FCRL4+ B cells from viraemic HIV-positive patients, it is not known whether the cells also home to lymphoid organs. Although CD27 is shown only on FCRL4− B cells, a fraction of FCRL4+ memory B cells also expresses CD27.4, 5 HIV, human immunodeficiency virus.