Antibody repertoire development in fetal and neonatal piglets. IX. Three pathogen-associated molecular patterns act synergistically to allow germfree piglets to respond to type 2 thymus-independent and thymus-dependent antigens.

Newborn piglets maintained germfree (GF) cannot respond to either thymus-dependent (TD) or type 2 thymus-independent Ags (TI-2) unless colonized with bacteria. We show here that pathogen-associated molecular patterns (PAMPs), including muramyl dipeptide (MDP), LPS, and a B-class CpG oligonucleotide (CpG-B), can substitute for gut flora in the induction of neonatal immunoresponsiveness. These PAMPs alone or in combination had little effect on serum IgG and IgA levels, but CpG-B and CpG-B + MDP elevated total IgM levels 3- to 7-fold above that seen in colonized controls after booster immunization. Although only CpG-B could alone stimulate immunoresponsiveness, co-administration of LPS or MDP resulted in a 5-fold increase in the IgG response to both immunogens. Co-administered MDP did not promote secondary IgG responses to either Ag but instead pronounced secondary IgM responses to the epitopes of both immunogens. LPS co-administered with CpG-B may promote class switch recombination or cause differentiation of previously switched cells that become responsive after exposure to CpG-B. Primary and secondary IgG responses equally recognized the epitopes of the TI-2 and TD immunogens, whereas IgM responses favored the TI-2 epitope. Because PAMPs alone can result in Abs to 2,4,6-triitrophenyl and FLU without immunization, it suggests they alone cause differentiation of B cells of the preimmune repertoire. The finding that both bacterial PAMPs and colonization are capable of stimulating Ab responses in both immunized and nonimmunized piglets suggests that PAMPs derived from host flora may play a major role in awakening adaptive immunity in neonates.