Human innate B cells: a link between host defense and autoimmunity?

Abstract

B cells play a variety of immunoregulatory roles through their antigen-presentation ability and through cytokine and chemokine production. Innate immune activation of B cells may play a beneficial role through the generation of natural cross-reactive antibodies, by maintaining B cell memory and by exercising immunomodulatory functions that may provide protection against autoimmunity. In this article, we review human B cell populations and their functional properties, with a particular focus on a population of inherently autoreactive B cells, which seem to play an important physiological role in innate immunity, but which, if selected into adaptive immune responses, appear to become pathogenic agents in systemic lupus erythematosus.

Fig. 1

Human B cell subsets. Representative examples of human B cell populations are shown. a Analysis of normal tonsils by flow cytometry using antibodies against IgD and CD38 reveals the following subsets: Bm1 (virgin naïve, IgD⁺⁺CD38⁻); Bm2 (antigen-experienced naïve, IgD⁺⁺CD38⁺); Bm2’ (pre-GC, IgD⁺CD38⁺⁺); Bm3 (centroblast, IgD⁻CD38⁺⁺); Bm4 (centrocyte, IgD⁻CD38⁺⁺); Bm5 (memory, IgD⁻CD38^+/−). b Similar analysis of peripheral blood using IgD and CD27 markers. Healthy subjects demonstrate two dominant populations, naïve (IgD⁺, CD27⁻) and memory (identified as CD27⁺). In turn memory cells can be classified into either IgD⁺/IgM⁺, IgM-only (not shown) and isotype switched. This type of analysis also reveals a very minor fraction of IgD/CD27 double negative B cells (DN). Our preliminary studies suggest that DN B cells contain CD27⁻ memory cells and can be induced to acquire CD27 expression under some culture conditions that include stimulation with CpG DNA (Cappione et al., in preparation). DN B cells, which usually go undetected in normal subjects due to their low frequency, are frequently greatly expanded in patients with active SLE in whom they sometimes represent the dominant peripheral blood subset []. Similarly, circulating CD27^High plasmablasts which are quite scarce in healthy subjects are frequently increased in active SLE. c Characterization of B cell subsets in normal human spleen using staining for CD21/CD23, IgM/IgD. This type of staining generates a pattern very similar to the one obtained with mouse spleen and permits separation into follicular (FO), marginal zone (MZ) and transitional B cell populations (T1/T2). An additional subset can be seen in human spleen that appears to represent an IgD⁺ marginal zone population (MZ*) (SLE systemic lupus erythematosus)

Fig. 2

Recognition of apoptotic cells by 9G4 antibodies. a Dot plots in the left panels demonstrate induction of apoptosis in a CD45⁻ Jurkat cell line treated with anti-Fas antibodies. The histograms shown in the right panels demonstrate that 9G4 antibodies (derived in this example from lupus serum) to apoptotic cells but not to viable cells after anti-Fas treatment. b The binding of 9G4 antibodies to apoptotic cells was corroborated by immunofluorescence. In this example, cells were incubated with either AnnexinV-PE or 9G4-FITC after 18 h of anti-Fas treatment. Only AnnexinV⁺ cells were also stained with 9G4, indicating specific binding to apoptotic cells

Fig. 3

Histological distribution of 9G4 B cells. a Representative examples of GC from normal tonsils are shown. While 40% of all GC were positive for expansions of Lc1⁺ B cells (representing control VH4 B cells), autoreactive 9G4 do not form productive GC. b In normal spleen, 9G4 B cells accumulate in the follicular mantle (FM) and can also be seen within the anatomical marginal zone (MZ) but are absent from the GC proper. c Staining of tonsil biopsy specimens obtained from patients with SLE demonstrates mature GC reactions formed by expansions of 9G4 B cells (GC germinal center)

Fig. 4

Expression of CD1 antigens in human B cells. Total CD19⁺ human tonsil B cells were stained with antibodies against CD1b and CD1d as well as 9G4. Both CD1b and CD1d are expressed in virtually 100% of 9G4 B cells with higher relative levels of CD1d. The universal expression of CD1b by 9G4 cells is in contrast to total B cells, which can be divided into CD1b-positive and -negative fractions

Fig. 5

Hypothetical model for the regulation of autoreactive B cells. In healthy subjects, autoreactive 9G4 B cells are functionally sequestered within the innate immune system and participate in beneficial immune responses in a GC-independent, T cell-independent manner that prevents their accumulation in the long-lived IgG compartments. In contrast, in SLE intrinsically abnormal B cells respond to a number of possible stimuli (infections, excessive BAFF, excessive IFN-α, abnormal T cell regulation) by participating in productive GC reactions and expanding into the IgG post-GC memory and plasma cell compartments