Expression of Ia Antigen on Adult and Neonatal B Lymphocytes Responsive to Thymus-Independent Antigens

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Abstract
Spleen cells from normal adult or neonatal mice were treated with anti-Ia and C and in vitro responses to two thymus-independent (TI) antigens, trinitrophenyl-Brucella abortus (TNP-BA) and TNP-Ficoll were studied. TNP-BA, a TI antigen that is known to stimulate neonatal B cells, was used in these experiments with the intent of stimulating an early appearing population of B lymphocytes, which might be anticipated to express surface immunoglobulin (sIg+) but not Ia. Cells treated with C and antisera with activity to I region determinants coded for by genes in I-A, I-E, or I-E/C were unable to mount significant responses to TNP-BA or TNP-Ficoll. These diminished responses could not be reconstituted with x-irradiated cells as a source of macrophages. When neonatal spleen cells, which allegedly possess a high fraction of Ig+ Ia- cells, were treated with A.TH anti-A.TL, there was a marked impairment in their ability to respond in vitro to TNP-BA. Thus, even primitive TI responses depend largely on cells that have already expressed Ia. To assess whether spleen cells treated with anti-Ia and C contained small numbers of immunocompetent cells that might not have been detected in in vitro systems employed, we transferred antisera and C-treated (CBA/N × DBA/2)F1 female spleen cells to B cell defective (CBA/N × DBA/2)F1 male mice and challenged the recipients with TNP-Ficoll 7 or 14 days later. Anti-TNP PFC were measured 5 days after antigen challenge. There was no detectable anti-TNP response seen in F1 male mice reconstituted with anti-Ia + C-treated cells whereas cells treated only with C were able to transfer significant responses. Responses to TNP-Ficoll were shown to be more sensitive to anti-Ia and C treatment than were responses to TNP-BA suggesting that the subset of B cells responding to TI-2 antigens may bear a higher density of surface Ia than does the B cell subset responding to TI-1 antigens.