The Role of Endogenous Interleukin-12 in Resistance to Murine Cytomegalovirus (MCMV) Infection and a Novel Action for Endogenous IL-12 p40

Biologically active interleukin-12 (IL-12), comprising a 40 kDa subunit (p40) covalently linked to a 35 kDa subunit (p35), is produced in response to a range of infectious stimuli. Here, we demonstrate that mice deficient in either IL-12 p40 (p40^-/-) or IL-12 p35 (p35^-/-) are susceptible to murine cytomegalovirus (MCMV) infection in terms of survival (Balb/c p35^-/-) and viral clearance (Balb/c p35^-/- and Balb/c p40^-/-), and this susceptibility may be correlated to a deficiency in serum interferon-γ (IFN-γ) levels. These data support a role for endogenous IL-12 in controlling MCMV infection. The IL-12 p40 subunit is produced in excess of IL-12 p35, and to date the function of the excess endogenous p40 has been assumed to be one of IL-12 antagonism, as demonstrated by experiments with exogenous p40 both in vivo and in vitro. We show that Balb/c p35^-/- alone are significantly compromised in survival of a sublethal infection and in clearance of virus from the spleen. These mice produce a very early IFN-γ spike (8 h after infection) and an aberrant tumor necrosis factor-α (TNF-α) spike (day 2 after infection). MCMV infection has revealed an altered Balb/c p35^-/- phenotype compared with Balb/c p40^-/-, and this indicates that endogenous p40 may have an activity independent of and additional to IL-12 antagonism in vivo.