Cooperative IFN-γ production of mouse liver B cells and natural killer cells stimulated with lipopolysaccharide

Background/Aims

Although mouse liver contains a large population of B cells, little is known about how hepatic B cells respond to bacterial lipopolysaccharide (LPS).

Methods

The cytokine and IgM productions of hepatic B cells were compared with those of splenic B cells. The effect of LPS-treated hepatic B cells on IFN-γ production from co-cultured NK1.1⁺ cells was also examined by irradiation and transwell experiments.

Results

Hepatic B cells stimulated with LPS produced substantial amounts of IFN-γ and IL-12 but a small amount of IgM, while splenic B cells did not produce any of these cytokines but produced a large amount of IgM. The hepatic B cells expressed surface markers similar to those on spleen B cells but expressed more C–X–C chemokine receptor 3 than spleen B cells. Notably, depletion of B220⁺ cells from liver MNCs (but not from spleen MNCs) greatly decreased LPS-induced IFN-γ production. Furthermore, LPS-treated hepatic B cells stimulated liver NK1.1⁺ cells to produce a remarkable amount of IFN-γ, not only through their soluble factors but also through direct cell–cell contact.

Conclusions

Liver B cells may play an important role in the defense against gram-negative bacterial infections by inducing IFN-γ production from liver NK cells.

Introduction

It is well known that B cells respond to bacterial components, such as lipopolysaccharide (LPS), in the absence of MHC-class II-restricted T-cell help. These T-independent antigens directly activate B cells and trigger them to proliferate and secrete IgM [1]. Recent studies have shown that activated B cells come to have the ability to produce IFN-γ under further stimulation such as from IL-12 or IL-18 [2], [3], [4], [5], suggesting that B cells are not only important for humoral immune response but are also relevant for cellular immune response.

The liver is an important immune organ for the T helper (Th) 1 response and innate immunity in the host defense against bacterial infections and tumors [6], [7], [8], [9]. Mononuclear cells (MNCs) from the liver contain a large population of natural killer (NK) and NKT cells having a potent capacity to produce IFN-γ [6], [9], [10], [11], [12]. Bacterial components or products, such as lipopolysaccharide (LPS) and staphylococcal enterotoxins, strongly activate liver NK and NKT cells when they are injected into mice, producing IFN-γ via IL-12 produced by Kupffer cells [13], [14]. This is also the case in bacterial infections themselves [6], [12], [13].

Recent studies have shown that mature B cells preferentially express the LPS signaling receptor RP105 (CD180), which is a member of the toll-like receptor (TLR) family [15], [16], [17]. The LPS-induced cell proliferation and Ig production of B cells from RP105-deficient mice are severely impaired [16], [18]. TLR-4 is also well known as an LPS signaling receptor that is expressed on granulocytes, B cells, and macrophages [19], [20], and the proliferation of B cells in TLR-4-deficient mice in response to LPS is also severely impaired [21]. These findings suggest that LPS can activate/stimulate B cells via LPS signaling receptors.

On the other hand, we have been aware and others have reported [22], [23] that the liver MNCs contain a substantial number/proportion of B cells. However, how liver B cells behave in response to LPS is unknown. In the present study, we investigated the function of liver B cells in the liver stimulated with LPS, focusing on their Th1 cytokine production and co-operation with NK cells.