Detailed analyses of antibodies recognizing mitochondrial antigens suggest similar or identical mechanism for production of natural antibodies and natural autoantibodies.

Because of their endosymbiotic evolutionary origin, proteins compartmentalized into mitochondria represent an interesting transition from prokaryotic foreign to essential self molecules. We investigated the presence of naturally occurring antibodies (nAbs) recognizing mitochondrial inner membrane enzymes. Epitope mapping analysis of a mitochondrial inner membrane enzyme, citrate synthase (CS) by synthetic overlapping peptides and phage display libraries using sera from healthy individuals and from patients having systemic autoimmune disease revealed CS recognizing nAbs with IgM isotype. We analyzed cross-reactive epitopes on human CS, bacterial CS, and various standard autoantigens. We have found that the fine epitope pattern on CS is different under physiological and pathological conditions. Moreover sera affinity purified on CS cross reacts with nucleosome antigen, which cross-reactivity could be mapped to a short epitope on human CS. These data indicate that in theory, nAbs "specific" for a given self antigen could fulfill the function of participating in innate defense mechanisms and at the same time recognize a target antigen in a systemic autoimmune disease. Thus, at the level of recognized epitopes there is a possible link between the innate like part and the adaptive-autoimmune arm of the humoral immune system.

Complex organizational defects of fibroblast architecture in the mouse spleen with Nkx2.3 homeodomain deficiency.

The capacity of secondary lymphoid organs to provide suitable tissue environment for mounting immune responses is dependent on their compartmentalized stromal constituents, including distinct fibroblasts. In addition to various members of the tumor necrosis factor/lymphotoxin beta family as important morphogenic regulators of peripheral lymphoid tissue development, the formation of stromal elements of spleen is also influenced by the Nkx2.3 homeodomain transcription factor in a tissue-specific fashion. Here we extend our previous work on the role of Nkx2.3-mediated regulation in the development of spleen architecture by analyzing the structure of reticular fibroblastic meshwork of spleen in inbred Nkx2.3-deficient mice. Using immunohistochemistry and dual-label immunofluorescence we found both distributional abnormalities, manifested as poor reticular compartmentalization of T-zone and circumferential reticulum, and developmental blockade, resulting in the absence of a complementary fibroblast subpopulation of white pulp. The disregulated distribution of fibroblasts was accompanied with an increased binding of immunohistochemically detectable complement factor C4 by T-cell zone-associated reticular fibroblasts, distinct from follicular dendritic cells with inherently high-level expression of bound C4. These data indicate that the impact of Nkx2.3 gene deficiency on fibroblast ontogeny within the spleen extends beyond its distributional effects, and that the formation of various white pulp fibroblast subsets is differentially affected by the presence of Nkx2.3 activity, possibly also influencing their role in various immune functions linked with complement activation and deposition.