The chemical nature of macrophage RNA-antigen complexes and their relevance to immune induction

Gene activation during immune reaction

In cell-free systems the addition of antigen stimulates the synthesis of informational RNA (i-RNA) which exhibits the following properties: It codes for the entire antibody molecule, it codes for the synthesis of regulator protein which initiates transcription of i-RNA with the correspondent informational content from DNA, it is a template for an an i-RNA dependent RNA polymerase, it is a template for an i-RNA dependent reverse transcriptase. The i-RNA may exist in a state of latency in cells. The product of reverse transcription of i-RNA is i-DNA which can be used to transcribe further i-RNA of the same specificity. Similar to i-DNA is an extracellular DNA which codes also for antibody and from which i-RNA can be transcribed. The data presented are summarized in a scheme of the flow of information during immunological reactions. It could be shown that there exist three different types of extrachromosomally synthesized molecules--i-RNA, i-DNA and extracellular DNA--which bear immunological specific information. These extrachromosomal states of information may be relevant for the generation of antibody diversity.

Preliminary characterization of "immunogenic" ribonucleic acid derived from rat peritoneal exudate cells

An "immunogenic" ribonucleic acid (Im-RNA) has been extracted from peritoneal exudate (PE) cells of rats that were immunized with sheep erythrocytes (SRBC). Following multiple phenol extractions and deoxyribonuclease treatment, the material obtained from PE cells was eluted from diethylaminoethyl-cellulose at 0.55 M NaCl concentration and partially purified in this procedure by a factor of 7- to 10-fold. After column chromatography, Im-RNA was found to be free of antigen based on results using (51)Cr-labeled SRBC or (14)C-dinitrophenol coupled to methylated bovine serum albumin as antigens. The Im-RNA showed a biphasic hyperchromicity curve when heated. The first phase, from 30 C to 90 C was gradual, accounting for 15.2% hyperchromicity suggestive of transfer RNA melting. No loss in immunogenic activity was observed when the Im-RNA was heated to 90 C. The second phase, from 90 C to 102 C, accounting for 15.2% further hyperchromicity, had a calculated T(m) of 96 C. Heating above 90 C resulted in an irreversible loss of immunogenic activity. These results strongly suggested that the RNA fraction contained a highly ordered secondary structure such as might be found with double-stranded nucleic acid. The nature and function of the Im-RNA is discussed.

The interaction of soluble horseradish peroxidase with mouse peritoneal macrophages in vitro

The in vitro interaction of soluble horseradish peroxidase (HRP) with homogeneous mono layers of mouse macrophages has been studied using sensitive biochemical and cytochemical techniques. The compartmentalization of HRP in extracellular and intracellular sites has been quantitatively evaluated. A significant fraction is bound to a serum-derived layer, which coats the surface of culture vessels and may be removed by appropriate washes. Macrophages interiorize HRP as a solute in pinocytic vesicles without appreciable binding of the glycoprotein to the plasma membrane. Uptake is directly proportional to the concentration of HRP in the culture medium. 1 x 10(6) cells ingest 0.0025% of the administered load per hr over a wide range of concentrations. Cytochemically, all demonstrable HRP is sequestered within the endocytic vesicles and secondary lysosomes of the vacuolar apparatus. After uptake, the enzymatic activity of HRP is inactivated exponentially with a half-life of 7-9 hr, until enzyme is no longer detectable. When macrophages have pinocytosed trace-labeled HRP-(125)I, cell-associated isotope disappears with a t (1/2) of 20-30 hr and they release monoiodotyrosine-(125)I into the culture medium. We were unable to obtain evidence that significant amounts of HRP (>2%) can be exocytosed after uptake, can exist intact on the cell surface, or can be digested extracellularly. It is difficult to reconcile these observations with several of the postulated mechanisms whereby macrophages are thought to play a prominent role in the induction of an immune response.

Potentiation of the T-lymphocyte response to mitogens. II. The cellular source of potentiating mediator(s)

Effective supernatants (SUP), which potentiate mouse T-cell responses to phytohemagglutin (PHA), are obtained from cells of several species (human, rabbit, rat, mouse) and indeed from syngeneic spleen, thymus, or bone marrow cells. Unstimulated cells release some SUP activity but more is produced after stimulation. Lipopolysaccharide (LPS) produced very active SUP in all cultures tested. PHA was similarly active on human leukocytes only, whereas concanavalin A (Con A) gave highly efficient SUP only with mouse spleen cells. SUP production is not correlated with a mitotic response of the donor cells and is observed in cultures unable to respond mitotically to the stimulant. Adherent mouse spleen cell populations, consisting largely or entirely of macrophages, produce active SUP, while nonadherent cells do not. Similarly, purification of human peripheral leukocytes on nylon columns, with removal of macrophages and other adherent cells, destroys their ability to produce SUP. The importance of indirect effects in stimulating mitotic responses of T cells is emphasized by the fact that the mitotic response of mouse thymocytes to LPS and its ability to potentiate the response of these cells to PHA disappears with removal of adherent cells from the thymocyte population. Conversely the production of SUP from spleen cells stimulated by Con A requires the presence of T cells.

Uptake of radioiodinated antigens by human monocytes

The capacity for human monocytes to take up tuberculin purified protein derivative (PPD) and tetanus toxoid (TT), as well as human serum albumin, transferrin and IgG was studied in vitro. PPD was taken up more avidly and lost from the cells more rapidly than were the other proteins studied. The major fraction of PPD was rapidly degraded. Serum factors inhibited the uptake of the bacterial antigens: PPD uptake was reduced by IgG, albumin and serum, whereas TT uptake was significantly reduced only by whole serum. Although selective binding of the antigens to serum proteins occurred, the binding patterns did not correspond to the patterns of inhibition of antigen uptake induced by serum of its fractions.

Monocytes from patients with active tuberculosis took up significantly less tubercular antigen than did monocytes from normal subjects or patients with inactive tuberculosis.

The immunogenicity of antigen bound to the plasma membrane of macrophages

Macrophages were cultured for several hours after a brief exposure to radio-iodinated keyhole limpet hemocyanin. Most of the hemocyanin taken up by the macrophages was rapidly catabolized and eliminated from the cell. A few molecules were retained on the plasma membrane of the cells for prolonged periods and were not subject to endocytosis and catabolism. These few molecules of hemocyanin bound to the plasma membrane were identified by observing the fixation of antibody fragments to macrophages at low temperature. The membrane-bound antigen, which could be removed by trypsin or EDTA, was of large molecular size, though heterogeneous. A great part of the immune responses of mice to hemocyanin bound to live macrophages could be abrogated by treatment of the macrophages in vitro with antibody or trypsin. Hence, most of the immunogenicity of hemocyanin bound to macrophages was attributed to the few molecules of antigen bound to the plasma membrane.