Immunoglobulin switch circular DNA in the mouse infected with Nippostrongylus brasiliensis: evidence for successive class switching from mu to epsilon via gamma 1.(1)
As B lymphocytes differentiate into memory or plasma cells, the isotype of immunoglobulin heavy (H) chains changes from kL to one of the other isotypes, g1, g2a, g2b, g3, E, and a in the mouse. Infection by the larvae of the nematode parasite Nippostrongylus brasiliensis causes an approximately 100-fold increase in serum IgE concentration. The parasite can trigger a series of events responsible for the isotype switch. The lymphokine interleukin 4 (IL-4) is known to be one of the key molecules which mediates class switching in the parasite-infected mouse. In culture IL-4 was shown to promote switching from M to G1 or to E when splenic B cells were stimulated by a mitogenic lipopolysaccharide (LPS).
For each experiment 10 mice were infected with 700 larvae of N. brasiliensis. Cells were obtained from mesenteric lymph nodes 8, 10, or 12 days after inoculation. Splenocytes were obtained from BALB/c mice and cultured in RPMI 1640 medium supplemented with 10% fetal bovine serum, 50 AuM 2-mercaptoethanol, and antibiotics. Cells (5 x 10^8 cells) were stimulated with Salmonella typhosa LPS at 40 ug/ml, and recombinant IL-4 at 250 or 1000 units/ml. After 3 days of culture, IL-4-stimulated cells were recovered and examined for their surface immunoglobulin expression.
Since the serum IgG1 and IgE start to appear 10 days after the infection, samples were obtained on days 8, 10, and 12.
When treated with IL-4 (250 units/ml), 24% of cells became surface G1-positive, while E-positive cells were less than 0.1%. With a higher concentration of IL-4 (1000 units/ml), 1.5% of cells were positive for surface E. As in the in vivo experiments, two types of switch circle clones were identified in the IL-4 culture.
Two types of switch circular DNA were identified in these switch systems. One type is the G1 circle (5′-Sg1-positive), and the other is the E circle (5′-Se positive). The IgG1-positive cells could be immediate precursors to the IgE-producing cells. Switch circles started to appear 1-2 days before the production of IgG1 and IgE in the infected mouse.