In BALB/c mice, IL-4 production during the initial phase of infection with Leishmania major is necessary and sufficient to instruct Th2 cell development resulting in progressive disease

In contrast to intact BALB/c mice, BALB/c mice rendered deficient in Vβ4⁺ CD4⁺ T cells develop a Th1 response to infection with Leishmania major and are resistant. Vβ4-deficient BALB/c mice are unable to generate the early IL-4 transcription occurring in Vβ4 Vα8 CD4⁺ T cells of BALB/c mice within 1 day of infection. Here we demonstrate that treatment of Vβ4- deficient BALB/c mice with IL-4 during the first 64 h after infection instructs Th2 cell development and susceptibility to infection. The demonstrated inability of IL-4 to reverse the resistant phenotype of BALB/c mice treated with anti-CD4 mAb the day before infection suggest that these effects of IL-4 require its interaction with CD4⁺ T cells. In contrast to draining lymph node cells from BALB/c mice, cells from Vβ4-deficient BALB/c mice remain responsive to IL-12 following infection. Strikingly, administration of IL-4 to Vβ4-deficient BALB/c mice renders their lymph node cells unresponsive to IL-12 by down-regulating IL-12R β2-chain expression. This study directly demonstrates that in BALB/c mice IL-4 is necessary and sufficient to initiate the molecular events steering Th2 cell maturation and susceptibility to L. major.