RNA splicing is an increasingly recognized regulator of immunity. Here, we demonstrate that after Mycobacterium tuberculosis infection (mRNA) il12rb1 is spliced by dendritic cells (DCs) to form an alternative (mRNA) il12rb1Deltatm that encodes the protein IL-12Rbeta1DeltaTM. Compared with IL-12Rbeta1, IL-12Rbeta1DeltaTM contains an altered C-terminal sequence and lacks a transmembrane domain. Expression of IL-12Rbeta1DeltaTM occurs in CD11c(+) cells in the lungs during M. tuberculosis infection. Selective reconstitution of il12rb1(-/-) DCs with (mRNA) il12rb1 and/or (mRNA) il12rb1Deltatm demonstrates that IL-12Rbeta1DeltaTM augments IL-12Rbeta1-dependent DC migration and activation of M. tuberculosis-specific T cells. It cannot mediate these activities independently of IL12Rbeta1. We hypothesize that M. tuberculosis-exposed DCs express IL-12Rbeta1DeltaTM to enhance IL-12Rbeta1-dependent migration and promote M. tuberculosis-specific T cell activation. IL-12Rbeta1DeltaTM thus represents a novel positive-regulator of IL12Rbeta1-dependent DC function and of the immune response to M. tuberculosis.