Tam Dang

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.

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.

The role of NK cells in the development of autoantibodies

The systemic lupus erythematosus (Sle1) interval from the NZM2410 mouse strain has been shown to be responsible for high levels of autoantibody production against antinuclear antibodies (ANA) when transferred into C57BL/6 mice. B cells derived from the B6.Sle1 strain are required for the production but help from both T-dependent and independent sources have been documented. Using radiation chimeras constructed in a strain of mice that is chronically depleted of Natural killer (NK) cells, but not NKT cells, we have examined the role of NK cells in the development of ANA in this context. Our results show that in the presence of intact T cell help depletion of NK cells does not affect ANA production. However, when T cell help is compromised, the prevalence of animals producing ANA is significantly decreased suggesting that NK cells can provide help for the T-independent production of ANA. Further experiments provide a possible mechanism for the NK-cell dependence.

Regulation of μM VS μS mRNA expression in an inducible B cell line

Abstract During the course of B lymphocyte differentiation into immunoglobulin secreting cells the abundance of mRNA for the heavy chain of secreted IgM (μ s ) increases dramatically. In order to understand the regulatory events responsible for the selective increase in μ s mRNA we have looked for transcriptional alterations of VDJCμ gene segments as well as changes in the relative stability of μ m and μ s mRNA in BCL1 lymphoma cells which can be stimulated to increase the expression of μ s mRNA. These experiments showed that although the transcriptional level of the μ gene locus is not prefenentially augmented after stimulation, an alteration in the sites of polymerase termination is a significant factor contributing to the higher μ s to μ m ratio. This switch is dependent on new RNA synthesis. In addition, although the half-life of μ s mRNA is not selectively increased, stimulation of the cells does result in a specific enhancement of the half-lives of both species of μ mRNA, which accounts for the higher steady state levels of total μ message.