Evidence that IL-15 does not directly modulate the proliferation of MC38 cells. (A) No expression of IL-15Rα or IL-2/IL-15Rβ in MC38 cells. Expression of these molecules in MC38 cells was assessed by RT-PCR. Lanes: DNA marker (1-kb ladder; Invitrogen, Carlsbad, CA), MC38, CTLL2 (from left to right). Marker band size (from top to bottom) was 517/506 (doublet), 396, 344, 298, 220/201 (doublet). Due to the presence of multiple splice variants of the IL-15Rα transcript expressed by CTLL2 cells, at least 3 amplicons were detectable. In contrast, no band was amplified from MC38 mRNA. Amplification of the β-actin fragment from cDNA of MC38 or CTLL2 transcripts was confirmed as a loading control. Primer sets were IL-15Rα: SN, 5′-ATGGCCTCGCCGCAGCTCCGG-3′, AS, 5′-TAGAGATGGCCACTTTCGTCATTTTGG-3′; IL-2/IL-15Rβ: SN, 5′-CTT GGA GAT GCT GAT CCC TAG TAC-3′, AS, 5′-GGG AGC TCA GCT GGG AGA AGA ACT-3′. (B) Proliferative responses of MC38 cells and their IL-15Rα–transfected clones to IL-15. No significant proliferative responses were observed in the presence or absence of IL-15 with parental MC38 cells and their IL-15Rα–transfected clones.