Dorothy Yuan

Lipopolysaccharide deacylation by an endogenous lipase controls innate antibody responses to Gram-negative bacteria

T cell–independent type 1 agonists such as Gram-negative bacterial lipopolysaccharides can stimulate B lymphocytes to proliferate and produce antibodies by signaling through Toll-like receptors. This phenomenon is well established in vitro, yet polyclonal B cell responses after bacterial infection in vivo are often weak and short-lived. We show here that B cell proliferation and polyclonal antibody production in response to Gram-negative bacterial infection are modulated by acyloxyacyl hydrolase, a host enzyme that deacylates bacterial lipopolysaccharides. Deacylation of lipopolysaccharide occurred over several days, allowing lipopolysaccharide to act as an innate immune stimulant yet limiting the eventual amount of B cell proliferation and polyclonal antibody production. Control of lipopolysaccharide activation by acyloxyacyl hydrolase indicates that mammals can regulate immune responses to bacterial infection by chemical modification of a Toll-like receptor agonist.

Innate immune processes are sufficient for driving silicosis in mice

The lung is constantly exposed to potentially pathogenic particles and microorganisms. It has become evident recently that not only innate but also adaptive immune responses to particulates, such as SiO2 entering the respiratory tract, are complex and dynamic events. Although the cellular mechanisms and anatomical consequences involved in the development of silicosis have been studied extensively, they still remain poorly understood. Based on their capacity for immune regulation, lymphocytes may play a key role in the respiratory response to environmental challenge by SiO2. The objective of this study was to characterize the impact of SiO2 exposure on respiratory immune processes, with particular emphasis on evaluating the importance of lymphocytes in the murine silicosis model. Therefore, lymphopenic mice, including NK‐deficient, Rag1−/−, or a combination (Rag1−/− NK‐depleted), were used and demonstrated that SiO2‐induced fibrosis and inflammation can occur independently of T, B, NK T, and NK cells. Studies in Rag1−/− mice suggest further that lymphocytes may participate in the regulation of SiO2‐induced inflammation through modulation of the Nalp3 inflammasome. This observation may have clinical relevance in the treatment of inflammatory and fibrotic lung diseases that are refractory or respond suboptimally to current therapeutics.

Interactions between B lymphocytes and NK cells.

The ability of natural killer (NK) cells to secrete lymphokines confers upon them the potential to regulate cell types via mechanisms other than direct cytotoxicity. During the past few years increasing evidence has been accumulating to show that NK and B cells can interact productively. First, NK cells cocultured with B cells can induce them to initiate polyclonal Ig secretion. This help is mediated by a soluble factor (or factors) that appears to be different from any known cytokine. Second, preactivated B lymphocytes can induce NK cells to produce greater amounts of IFN‐γ via an interaction that requires direct cell contact. Third, in contrast to previous suggestions, NK cells do not have the ability to kill primary B lymphocytes regardless of their stage of differentiation. Evaluation of the in vivo relevance of these interactions revealed that activated NK cells can increase the IgG2a response to a specific protein antigen. Without activation, NK cells neither enhance nor inhibit B cell responses to antigens. The deviation of the isotype distribution may allow increased NK cell specificity for certain pathogens by enhancing antibody‐dependent cytotoxicity.—Yuan, D., Koh, C. Y., Wilder, J. A. Interactions between B lymphocytes and NK cells. FASEB J. 8: 1012‐1018; 1994.

IFN-γ-Dependent and -Independent Initiation of Switch Recombination by NK Cells

We have examined the effect of IL-2-propagated NK or NK-T cells on each of the steps required for B cell switch recombination leading to IgG2a production. The results indicate that NK cells, on their own and in the absence of IFN-γ, can induce germline transcription in resting, IgG− B lymphocytes from the γ2a locus as well as mRNA for activation-induced cytidine deaminase (AID) via a process that requires cell-cell interactions. The results also show that, in contrast to induction by T cells, activation by NK cells does not involve CD40-CD40 ligand interactions and does not extend to the induction of Iγ1 transcription. Furthermore, in contrast to stimulation by LPS and IFN-γ or by T cells, the activation events initiated by NK cells do not result in significant synthesis of functional γ2a mRNA in resting B lymphocytes even in the presence of IFN-γ. Thus, induction of germline and AID transcripts are necessary but not sufficient events for functional switching to IgG2a. These experiments, showing that NK cells themselves cannot induce IgG2a production but can polyclonally program B lymphocytes so that they preferentially switch to this isotype may explain how activated NK cells can skew the Ag-specific immune response toward IgG2a. The findings also provide further demonstration of the definitive yet limited extent of how a non-Ag-specific component of the innate system can modulate the direction of the adaptive immune response.

Distinct roles of adenylyl cyclase VII in regulating the immune responses in mice.

The second messenger cAMP plays a critical role in regulating immune responses. Although well known for its immunosuppressive effect, cAMP is also required for the development of optimal immune responses. Thus, the regulation of this second messenger needs to be finely tuned and well balanced in a context dependent manner. To further understand the role of cAMP synthesis in the functions of the immune system, we focus on a specific adenylyl cyclase (AC) isoform, AC VII (AC7), which is highly expressed in the immune system. We show that mice deficient of AC7 are hypersensitive to LPS-induced endotoxic shock. Macrophages from AC7-deficient mice produce more of the proinflammatory cytokine, TNF-α, in response to LPS. The inability to generate intracellular cAMP response to serum factors, such as lysophosphatidic acid, is a potential cause for this phenotype. Thus, AC7 functions to control the extent of immune responses toward bacterial infection. However, it is also required for the optimal functions of B and T cells during adaptive immune responses. AC7 is the major isoform that regulates cAMP synthesis in both B and T cells. AC7-deficient mice display compromised Ab responses toward both T cell-independent and T cell-dependent Ags. The generation of memory T cells is also reduced. These results are the first to ascribe specific functions to an AC isoform in the immune system and emphasize the importance of cAMP synthesis by this isoform in shaping the immune responses.

Specific killing of a human breast carcinoma cell line by a monoclonal antibody coupled to the A-chain of ricin

SummaryWe coupled monoclonal IgMk antibodies directed against human breast carcinoma cells to the A-chain of the plant toxin ricin. These molecular hybrids maintain both their antibody-binding activity and the toxic activity of the A-chain. Thus, they specifically bind to and kill the breast carcinoma cells in vitro.

Receptors and Counterreceptors Involved in NK-B Cell Interactions

In addition to the well-documented effect of NK cells on B cell differentiation via their ability to secrete IFN-γ, NK cells can also induce, via direct cell-cell interactions, germline transcripts (Iγ2a) necessary for switch recombination to IgG2a. Analysis of the ligand-receptor pairs that could be involved in this induction revealed that the expression of CD48 on B cells is crucial for the induction. NK cells from mice with targeted deletions of either the CD2 or the CD244 gene, both of which encode ligands for CD48, are compromised in their ability to induce B cell Iγ2a expression. Interestingly, although CD244 can bind to CD48 with a higher affinity, the ability of NK cells from CD244−/− mice to stimulate Iγ2a is not as compromised as NK cells from CD2−/− mice. Despite the difference between cell surface receptors that are stimulated by NK cells vs those stimulated by the combination of LPS and IFN-γ, we show in this study that the initiation of γ2a germline transcription is regulated by similar cis-acting elements located at the 3′ end of the IgH locus. However, NK cells cannot induce the final steps of switch recombination resulting in the production of mature mRNA from recombined DNA. Our findings suggest that these different signaling pathways converge on regulatory elements that are common to germline transcription; however, because NK induction does not result in the final steps of switch recombination, some signals initiated by LPS plus IFN-γ are not induced by NK cells.

Requirements for the natural killer cell-mediated induction of IgG1 and IgG2a expression in B lymphocytes

Upon interaction with resting B lymphocytes, IL-2-propagated NK cells can initiate the process of Ig constant region switch recombination (CSR) by inducing germ line transcripts for gamma2a (Igamma2a) as well as increased levels of mRNA for activation-induced cytidine deaminase enzyme. Whereas both these processes are necessary for CSR, they are not sufficient because the cells do not proceed to the expression of mature mRNA for gamma2a (VDJCgamma2a). In addition, NK cells can also upregulate mRNA for the T-box transcription factor (T-bet) in B cells without being able to induce further differentiation. Using transgenic B cells with B cell receptor specificity for nitrophenol (NP), we have now shown that NP-Ficoll-stimulated B cells can be induced by NK cells to express IgG2a as well as IgG1 presumably due to the completion of the process of switch recombination. The inductive ability of NK cells does not require IFN-gamma but does require signals transmitted via CD48 by direct cell contact. In addition, NP-Ficoll on its own can induce proliferation of antigen-specific B cells as well as germ line transcripts of gamma1; however, expression of VDJCgamma1 mRNA also requires NK cell interaction with B lymphocytes. Therefore, in the presence of antigen, NK cells can provide a necessary signal that substitutes for cytokines in the induction of IgG2a as well as IgG1 expression. This in vitro analysis provides a mechanistic basis for understanding the documented NK cell effects on T-independent B cell responses in vivo.

Interactions between NK cells and B lymphocytes.

Publisher Summary This chapter focuses on interactions between natural killer (NK) Cells and B Lymphocytes. Through studies of the extent of NK cell participation during the early phases of infection by a number of organisms, NK cells have long been well established as being an important element of innate immunity that can boost immune resistance before the development of specific immunity. They are also important in shaping the nature of the B- and T-cell responses. Natural killer cells constitute one of the key components of the innate immune system in that they can be rapidly activated without the need for expansion of antigen-specific clones. B lymphocytes, by contrast, are important constituents of the specific immune system because of the presence of clonally distributed antigen receptors and the persistence of the progeny of specific clones that can continue to produce the relevant antibodies. The most direct way of demonstrating that NK and B cells can interact is to isolate purified populations of each cell type, coculture them, and determine the effect of these interactions on parameters of cellular activation, such as induction of activation markers, proliferation, cytokine production, and in the case of B lymphocytes, immunoglobulin secretion, and isotype switching.

B Cell Induction of IL-13 Expression in NK Cells: Role of CD244 and SLAM-Associated Protein

NK cells are an important component of the innate immune system that can also interact with B cells in a mutually productive manner. We have previously shown that activated B cells can induce NK cells to up-regulate their secretion of IFN-γ. In this study, we show that B cells, and, particularly, marginal zone B cells, can, in addition, induce NK cells via direct cell-cell interactions to express mRNA encoding the Th2 cytokine IL-13. The induction of NK cell IL-13 mRNA expression requires the ligation of the CD244 receptor by the CD48 ligand on B cells via signaling pathways that depend upon expression of the X-linked lymphoproliferative disease gene product, SH2D1A/DSHP/SAP (SLAM-associated protein, or SAP) in NK cells. Thus, the positive signals attributed to the B cell activation of CD244 on murine NK cells appears to be more similar to the activity of CD244 on human cells. The induction of IL-13 mRNA by B cells may account for the effect of NK cells on the generation of Th2-type responses in the presence of some adjuvants.