IL-15 is normally not secreted in soluble form (8-10) but is held on the cell surface bound to a unique receptor, IL-15R␣, especially on dendritic cells (11-16). Cell-bound IL-15 then is presented in trans to T cells and NK cells and is recognized by the ␥ c receptor on these cells; such recognition maintains cell survival and intermittent proliferation.IL-15R␣ plays a mandatory role in presenting endogenous IL-15. Thus, like IL-15 -/-mice (1), IL-15R␣ -/-mice lack CD122 hi CD8 ϩ cells and NK cells (17), presumably because the IL-15 synthesized in IL-15R␣ -/-mice fails to leave the cytoplasm. Nevertheless, ␥ c ϩ cells can proliferate in response to a soluble recombinant form of IL-15 in the absence of IL-15R␣ (18). Moreover, under certain conditions, IL-15R␣ can be inhibitory. Thus, injecting mice with a soluble (s) recombinant form of IL-15R␣ is reported to suppress NK cell proliferation (10) and certain T dependent immune responses in vivo (19)(20)(21)(22), and adding sIL-15R␣ in vitro can block the response of cell lines to . Despite these findings, there are other reports that sIL-15R␣ (26), and also a soluble sushi domain of IL-15R␣ (27), can enhance IL-15 responses of human cell lines.In this paper, we investigated whether sIL-15R␣ can alter the response of normal mouse T cells to IL-15. As discussed below, IL-15 responses of CD8 ϩ T cells and NK cells are improved considerably by association with sIL-15R␣, both in vitro and in vivo.
Results
Stimulation by IL-15͞IL-15R␣ Complexes in Vitro.To examine whether the stimulatory function of soluble IL-15 is altered by binding to sIL-15R␣, purified MP CD44 hi CD122 hi CD8 ϩ cells were cultured in vitro with mouse IL-15 Ϯ mouse sIL-15R␣ covalently linked to the Fc portion of human IgG1 (sIL-15R␣-Fc). For IL-15 alone, half-maximal responses required Ϸ30 ng͞ml and responses were negligible with Ͻ10 ng͞ml (Fig. 1A and B). Here, the notable finding was that supplementing a low concentration of IL-15, e.g., 5 ng͞ml, with sIL-15R␣-Fc led to strong proliferative responses of MP CD8 ϩ cells as measured either by carboxyf luorescein diacetate succinimidyl ester (CFSE) dilution (Fig. 1 A) or by [ 3 H]thymidine incorporation (Fig. 1B). No proliferation occurred with sIL-15R␣-Fc alone (Fig. 1B), and the addition of sIL-15R␣-Fc failed to alter the response of MP CD8 ϩ cells to a different cytokine, IL-2 (data not shown). With IL-15, sIL-15R␣-Fc did not appear to act by enhancing the half-life of IL-15 in vitro (Fig. 6, which is published as supporting information on the PNAS web site).With a limiting concentration of cytokine, IL-15 responses were improved generally by 6-to 9-fold by the addition of sIL-15R␣-Fc. Adding sIL-15R␣-Fc also considerably improved the IL-15 response of CD122 hi NK cells (Fig. 1C) but was relatively ineffective on MP (CD44 hi ) CD4 ϩ cells, which express intermediate levels of CD122 (Fig. 1C). Unexpectedly, sIL-15R␣-Fc plus IL-15 led to significant proliferation of typical naïve CD44 lo CD122 lo CD8 ϩ cells, although only with high concentrations o...
