DIFFERENTIAL EXPRESSION OF MESSENGER-RNA ENCODING INTERLEUKIN-12 P35 AND P40 SUBUNITS IN-SITU

Interleukin-12 (IL-12) is a heterodimeric cytokine that plays an impor tant role in the regulation of the immune response. For biological act ivity the expression of both subunits of IL-12, p35 and p40, is requir ed. Moreover, in the mouse the p40 chain of IL-12 specifically inhibit s the effects of the IL-12 heterodimer. In the present study we have a nalyzed by in situ hybridization the expression of the p35 and p40 mRN A in the spleens of BALB/c and mutant (SCID, nude, beige) mice, unstim ulated and after in vivo stimulation with lipopolysaccharide (LPS) and with staphylococcal enterotoxin B (SEB). In unstimulated spleens of B ALB/c mice, p35 and p40 mRNA were only detectable in a few strongly st ained single cells, p35 mRNA was expressed in addition weakly in the B cell areas. After injection of LPS or SEB, p40 mRNA was strongly indu ced in the T cell areas all over the spleen, whereas expression of p35 mRNA and its distribution pattern did not change. Surprisingly, most of the mRNA for p35 and p40 was localized in different areas of the sp leen and was apparently produced by different cells. In macrophage-dep leted spleens the increased expression of p40 mRNA in response to LPS was reduced but still detectable, demonstrating that other cells besid es macrophages can up-regulate IL-12 p40 mRNA. Nude mice showed a stro nger expression of p35 mRNA, SCID mice lacked the weak p35 staining of the B cell areas but showed a strong basal expression of both p35 and p40 mRNA and a focal response to LPS. The pattern of IL-12 mRNA expre ssion in beige mice was the same as in normal mice. These data demonst rate a spatial dissociation of expression of the two chains of IL-12 a nd are compatible with a regulatory role of the isolated IL-12 p40 cha in in vivo. In addition, they indicate that the demonstration of mRNA for both chains of IL-12 in whole tissues or cell mixtures is not nece ssarily indicative of functional IL-12.