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Heavy nuclear RNA contains high amounts of transcripts from repetitive sequences B1 and B2. Cytoplasmic poly(A)+RNA and, particularly, polysomal poly(A)+RNA (mRNA) also include these sequences but in smaller amounts. The abundant 2 kb mRNA of mouse liver are found to have a B2 sequence. These sequences are also found in a new class of low-molecular-weight heterogeneous (200-400 nucleotide long) poly(A)+RNAs. These RNAs are located mostly in cytoplasm rather than in polysomes. The amount of small B2+ RNAs is noticeable larger than that of small B1+ RNAs. Tumour cells seem to contain more small B2 RNA than normal cells. The hybridization tests show that extended homology exists between the B1 sequence and 4.5S small nuclear RNA, which is predicted from comparison of their base sequences. Also, we have found homology between B1 and small cytoplasmic 7S RNA. Hybridization of B2 to sn or sc RNAs has not been observed, although the sequencing reveals partial homology between B2 and 4.5S sn RNAI /1/.

Both free and hidden natural antibodies to DNA or cardiolipin were obtained from immunoglobulins of a normal donor. The free antibodies reacting with DNA or cardiolipin were isolated by means of affinity chromatography. Antibodies occurring in an hidden state were disengaged from the depleted immunoglobulins by ion-exchange chromatography and were then affinity-isolated on DNA or cardiolipin sorbents. We used flow cytometry to study the ability of free and hidden antibodies to bind to rat thymocytes. Simultaneously, plasma membrane integrity was tested by propidium iodide (PI) exclusion. The hidden antibodies reacted with 65.2 ± 10.9% of the thymocytes and caused a fast plasma membrane disruption. Cells (28.7 ± 7.1%) were stained with PI after incubation with the hidden antibodies for 1 h. The free antibodies bound to a very small fraction of the thymocytes and did not evoke death as compared to control without antibodies. The possible reason for the observed effects is difference in reactivity of the free and hidden antibodies to phospholipids. While free antibodies reacted preferentially with phosphotidylcholine, hidden antibodies reacted with cardiolipin and phosphotidylserine.

Immunoglobulin preparations from sera of healthy donors contain polyspecific autoantibodies interacting with DNA and other charged antigens. These antibodies belong to the IgG class and can exist in the free or hidden state. The hidden antibody activity can be revealed after ion-exchange chromatography on QAE-Sephadex A-50. Immunoenzyme assay was used to assess the interactions of both free and hidden antibodies with different carbohydrates. The hidden antibodies were only able to interact with different polyanionic carbohydrates and neutral xenoantigen Bdi.

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