Immunoglobulin switch circular DNA in the mouse infected with Nippostrongylus brasiliensis: evidence for successive class switching from mu to epsilon via gamma 1.

We have characterized immunoglobulin switch circular DNA in mice infected with the nematode parasite Nippostrongylus brasiliensis. Two kinds of circular DNA were identified in the lymph nodes as excision products of switch recombination of immunoglobulin heavy-chain constant region (CH) genes. One is a recombinant between C mu and C gamma 1 (gamma 1 circle), and the other is a recombinant between C gamma 1 and C epsilon (epsilon circle). In the epsilon circle, a short piece of switch mu (S mu) sequence was inserted between S epsilon and S gamma 1 sequences. The inserted S mu sequence could be a trace of the preceding switch from C mu to C gamma 1. These findings indicate that parasitic infection can induce class switch recombinations in a successive manner, first from C mu to C gamma 1, and then from C gamma 1 to C epsilon.