Immunoglobulin-containing cells and the origin of immunoglobulins in the respiratory tract of sheep.

The amounts of immunoglobulins formed in the respiratory tract of sheep were investigated by comparing the distribution of radiolabelled immunoglobulin between plasma and lymph from the caudal mediastinal lymph node efferent duct which drains 50-70% of lung lymph. In addition, immunoglobulin-containing cells in the lymph node lymph and in the respiratory mucosa were counted by immunofluorescence. Whereas 91.6 +/- 5.92% and 67.73 +/- 10.07% of IgG1 and IgG2 respectively were plasma-derived, only 35.89 +/- 7.09% of IgA was plasma-derived. IgM and albumin were wholly plasma-derived. There was no evidence for selective transport of any immunoglobulins from plasma into lymph. It was estimated that mediastinal lymph node lymph contributed 0.12 g IgA to the circulation daily, the bulk of this being of local origin. There were only few cells expressing IgA in the caudal mediastinal lymph node and its efferent lymph indicating that, unlike gut-associated lymphoid tissue, bronchus-associated lymphoid tissue in sheep is not a major site of IgA cell precursor production. Despite this finding, IgA was the most frequent isotype of Ig-expressing cells in the respiratory mucosa and it is concluded that the locally formed IgA in lymph node lymph originated from mucosal plasma cells. IgM- and IgG-expressing cells were much less numerous than IgA-cells in the mucosa, but were the predominant isotype in the caudal mediastinal lymph node and regional lymph nodes and in mediastinal lymph node efferent lymph. These experiments have established that the respiratory tract of sheep is qualitatively similar to the intestine with respect to immunoglobulin synthesis in that the bulk of IgA is locally derived and a smaller but significant local contribution of IgG1 and IgG2 occurs, but that it is a poor source of IgA cell precursors. This implies that the IgA plasma cell population in the respiratory mucosa probably originates from distant mucosal sites.