The results reported here show that B lymphocytes have a profound regulatory effect on the antigen-presenting function of DCs in vivo. Indeed, DCs from B cell–deprived animals have an impaired capacity to induce antigen-specific differentiation of IL-4–secreting T cells when transferred in control animals. The diminished IL-4–promoting capacity of DCs correlates with an enhanced production of IL-12. These observations suggest that B lymphocytes interact with DCs and lower the level of IL-12 released by DCs, thereby leading to priming of both Th1 and Th2 lymphocytes. Of note, it has been shown that interactions between DCs and B cells may occur regularly during B cell recirculation. The interaction is thought to be confined to small B cells, is totally T cell and antigen independent, and is not MHC restricted 20
. Furthermore, cooperation between DCs and B lymphocytes has been demonstrated by Dubois et al. 21
, who showed that in vitro–generated DCs promote the proliferation of naive and CD40-activated B cells and produce factors that induce differentiation of activated B cells into plasma cells. Our observations show that a bidirectional regulation occurs upon DC–B cell interaction.
Although we do not have direct evidence, several features suggest that IL-10 may be involved in this immunoregulatory process: splenocytes from μMT mice express reduced levels of IL-10 mRNA than wild-type animals; DCs from IL-10 knockout mice display properties similar to DCs from μMT mice, i.e., they have lost the capacity to induce the development of IL-4–secreting cells; treatment of DCs from μMT mice with IL-10 restored the generation of IL-4–producing cells in vivo; and numerous reports have shown that IL-10 inhibits the production of IL-12 heterodimer by DCs in vitro and in vivo 310
. Collectively, these observations suggest that the level of IL-12 released by DCs is regulated by B lymphocytes, presumably via the production of IL-10. Our data show that B lymphocytes may produce IL-10 constitutively. B cell–depleted spleen cells also express mRNA for IL-10 (data not shown), suggesting that other cell populations may indirectly control the level of IL-12.
There is evidence that the level of IL-12 released by transferred DCs determines the Th1/Th2 balance in vivo. DCs from wild-type animals produce intermediate levels of IL-12 and induce the development of T cells producing IL-2, IFN-γ, IL-4, IL-5, and IL-10 (this paper and reference 17). DCs, treated with IL-10 in culture, secrete decreased amounts of IL-12 and direct the differentiation of Th2-type cells 3
, similar to the response induced by DCs from IL-12–deficient mice 3
. Conversely, DCs from IL-10– or B cell–deficient mice release increased levels of IL-12 and do not promote IL-4 secretion (this paper). Coadministration of IL-12 with the wild-type DCs results in sensitization of Th1-type cells only 17
. Therefore, the development of IL-4–secreting T lymphocytes appears as a default when T cells are sensitized in the presence of low levels of IL-12. Whether IL-12 directly or indirectly inhibits the development of Th2-type cells is still a matter of speculation.
Our observations indicate that DCs from B cell–deprived mice have lost the ability to prime for IL-4 production but have retained the capacity to induce the development of cells producing diminished but still significant amounts of IL-5 and IL-10. The lack of association of expression among IL-4, IL-5, and IL-10 has been recently reported by Kelso et al. 22
, who analyzed the single-cell expression patterns in type 1– or type 2–polarized responses.
Our data also show that DCs from wild-type mice injected in a μMT host induce the development of T cells secreting IL-2 and IFN-γ, but not IL-4. Whether B lymphocytes limit IL-12 synthesis by transferred DC homing in the T cell area of lymph nodes remains to be determined. Additional studies, including injection of labeled DCs in the footpad and staining of lymph node sections for IL-12 p70, would help clarify this point. Interestingly, Wykes et al. 23
have shown that DCs can retain unprocessed antigen and directly interact with B lymphocytes to initiate antibody synthesis. Thus, B cells recognizing the antigen expressed by DCs form short-lived clusters and could give regulatory signals to the transferred DCs. Alternatively, direct B to T cell signaling may promote IL-4 synthesis in this experimental model. It is noteworthy that T cells from μMT mice have the capacity to produce IL-4, as T cells from μMT mice produced high amounts of IL-4 in response to Schistosoma mansoni
or Onchocerca volvulus 24
. The capacity of T cells from μMT mice to secrete IL-4 is further illustrated by the observation that injection of DCs that have been treated with IL-10 to enhance their capacity to promote Th2 development 3
induces the production of significant levels of IL-4 in μMT recipient mice ().
We and others have reported that subclasses of DCs directed the development of distinct T helper cells in vivo 1819
. Thus, CD8α+
DCs, purified from spleens, have the potential to differentially regulate the Th1/Th2 balance: CD8α−
DCs induced the activation of cells secreting high levels of IL-4, IL-5, and IL-10, and low levels of IL-2 and IFN-γ, whereas CD8α+
DCs sensitize cells producing IL-2 and IFN-γ, but little IL-4, IL-5, or IL-10. We therefore compared the numbers of DCs of either subset in μMT and wild-type mice. Little difference was found between these strains, suggesting that B lymphocytes did not alter the distribution of DC subsets (). Of note, the number of DCs was consistently reduced (by approximately twofold) in B cell–deprived mice as compared with wild-type animals, an observation that may result from a lack of survival or maturation signals and/or chemoattractants factors such as B cell–derived chemokines macrophage inflammatory protein (MIP)-1α and MIP-1β 25
.Our results are consistent with the observations of Stockinger et al. 10
that the B cell is the crucial APC for the development of Th2 responses. Stockinger et al. also recently have identified a feedback loop triggered by IL-12 that promotes Th2 differentiation 26
. Thus, delivery of IL-12 by DCs during B cell activation induces the secretion of IL-6 and IL-10 by activated B cells. IL-6 and IL-10 confer the capacity to induce IL-4 expression in T cells to these B lymphocytes. These data were interpreted to indicate that DCs, through IL-12 secretion, enhance the ability of B cells to influence T cell differentiation toward Th2. Our data further extend these observations by showing that DCs themselves, after interaction with B cells, show higher IL-4–inducing activity.
Interestingly, our results may shed new light on the enigmatic observation that μMT mice showed enhanced CTL activity to tumor cells in comparison to their control littermates 27
. Qin et al. reported that the presence of B cells in the priming phase resulted in disabled help for CTL-mediated tumor immunity, and interpreted these data by a competition between B cells and other APCs for antigen. Our data suggest that the absence of B cells may enhance the generation of tumor immunity by promoting a polarized Th1 response.
In conclusion, our observations suggest that at the steady state level, IL-10 released by B lymphocytes may control the level of IL-12 released by DCs, which upon encounter with an antigen would induce an unpolarized immune response. In addition, there is evidence that presentation of antigen by B lymphocytes may preferentially direct the development of Th2-type cells 2829
. Therefore, B cells may directly (upon interaction with T cells) or indirectly (by controlling IL-12 production via DCs) favor the development of Th2-type cells that provide helper activity for antibody synthesis, thereby promoting their own effector function.