Identification of macrophage receptors for angiotensin: a potential role in antigen uptake for T lymphocyte responses?

To determine if macrophages express receptors for peptide antigens, guinea pig peritoneal exudate cells (PEC) were examined for their uptake of the octapeptide angiotensin II (AII). PEC were incubated with [3H]AII, with or without nonradioactive AII as a cold inhibitor, for varying lengths of time before harvesting to determine the cell-associated [3H]AII counts per minute. The PEC-associated [3H]AII increased from 90 to 120 min of incubation, then plateaued on additional incubation to 3.5 hr. Inclusion of nonradiolabeled AII into the culture decreased the cell-associated [3H]AII by 80 to 90% at all time points. The uptake of [3H]AII was temperature-sensitive, with maximum cell-associated [3H]-AII occurring at 37 degrees C and reduced uptake occurring at 4 degrees C. The association of [3H]AII with PEC was specific and saturable, and the inhibitory dose for reducing the cell-associated [3H]AII by 50% with nonradiolabeled AII was around 6 X 10(-6) M. Various AII analogs were also employed as inhibitors to determine the fine specificity of [3H]AII binding, and only those analogs with nonaromatic amino acid substitutions for the carboxyl terminal Phe8 showed reduced inhibitory activity, indicating that Phe8 is important for binding. Scatchard analysis of binding indicated that two classes of receptors interacted with AII: a low number of receptors with Ka approximately equal to 3.5 X 10(8) M-1, and a large number of relatively low affinity of binding showing a Ka approximately equal to 5 X 10(5) M-1. The cellular binding activity was associated with isolated PEC plasma membranes, and after density gradient fractionation of solubilized membranes, AII binding activity was primarily associated with molecules of m.w. of around 50,000. PEC were separated into macrophages and lymphocytes by adherence, and all of the [3H]AII binding activity was associated with the macrophage-enriched cells. These results show that macrophages express specific receptors for AII and related peptides that are responsible for most of the uptake of AII by macrophages. We discuss the relevance of this receptor for the immunologically important uptake of AII by macrophages, and a potential physiologic role in angiotensin-converting enzyme production.

Characterization of a 75,000 mol. wt glycoprotein synthesized by guinea-pig T-lymphocytes: a possible homologue of Lyt-1 antigen

A xenoantiserum raised in rabbits by immunization with strain 13 guinea-pig antigen-activated T-lymphocytes was previously found to recognize a non-immunoglobulin, 75,000 mol. wt glycoprotein synthesized by guinea-pig T-cells. This protein, p75, has been further characterized to determine its biochemical properties and its expression by various cell types. p75 was found to be a single-chain protein which could be bound by the lectin Lens culinaris hemagglutinin. It has an apparent mol. wt slightly greater than mu-chain as assessed by SDS-polyacrylamide gel electrophoresis and could not be precipitated by anti-guinea-pig immunoglobulin reagents. It exhibited considerable charge heterogeneity during isoelectric focusing and was not affected by neuraminidase treatment, p75 was synthesized by thymus, spleen and lymph node cells, by antigen-stimulated T-cells from strain 13 and strain 2 guinea-pigs, and by guinea-pig B-cell L2C leukemia lines, but not by normal B-lymphocytes or macrophages. No differences between the isoelectric focusing patterns of p75 molecules isolated from different cell types could be demonstrated. The chemical properties of p75 and its expression by the cell types so far examined indicate that p75 is a possible candidate for the guinea-pig homologue of the murine Lyt-1 antigen.

Evidence for covert cellular interaction sites expressed by activated T lymphocytes

In this report we examined the effects of trypsin treatment of immune guinea pig T lymphocytes on their binding to syngeneic macrophages. Immune T cells positively selected by culture for 1 wk with antigen and treated with trypsin showed dramatic binding to macrophages in the absence of additional antigen. This binding was rapid and greatest at 1 hr after the addition of trypsin-treated T cells to macrophages. Thereafter, the bound T cells dissociated from the macrophages and did not rebind. Dissociation of macrophage-bound T cells appeared to be an active process that could be inhibited by blocking T cell protein synthesis. Macrophage binding by trypsin-treated T cells was dependent on prior in vitro stimulation; similar treatment of immune T cells taken directly from the animal did not augment binding, and treatment of T cells cultured without antigen resulted in only modest binding. In addition, the trypsin-induced binding phenomenon showed elements of genetic restrictions. Trypsin treatment of immune T cells selected by culture for 1 wk with antigen resulted in preferential binding to syngeneic macrophages. In addition, immune F1 T cells selected by culture with antigen-pulsed parental macrophages preferentially bound to macrophages of the haplotype used for selection. Evidence is presented suggesting that the trypsin-induced binding was not simply due to antigen carry-over from the trypsinized lymphocytes and favoring the possibility that this binding may be antigen-independent. These results suggest that antigen-activated T cells express covert interaction sites for macrophages and that these sites are actively covered-up by other membrane components.

Evidence for covert cellular interaction sites expressed by activated T lymphocytes

In this report we examined the effects of trypsin treatment of immune guinea pig T lymphocytes on their binding to syngeneic macrophages. Immune T cells positively selected by culture for 1 wk with antigen and treated with trypsin showed dramatic binding to macrophages in the absence of additional antigen. This binding was rapid and greatest at 1 hr after the addition of trypsin-treated T cells to macrophages. Thereafter, the bound T cells dissociated from the macrophages and did not rebind. Dissociation of macrophage-bound T cells appeared to be an active process that could be inhibited by blocking T cell protein synthesis. Macrophage binding by trypsin-treated T cells was dependent on prior in vitro stimulation; similar treatment of immune T cells taken directly from the animal did not augment binding, and treatment of T cells cultured without antigen resulted in only modest binding. In addition, the trypsin-induced binding phenomenon showed elements of genetic restrictions. Trypsin treatment of immune T cells selected by culture for 1 wk with antigen resulted in preferential binding to syngeneic macrophages. In addition, immune F1 T cells selected by culture with antigen-pulsed parental macrophages preferentially bound to macrophages of the haplotype used for selection. Evidence is presented suggesting that the trypsin-induced binding was not simply due to antigen carry-over from the trypsinized lymphocytes and favoring the possibility that this binding may be antigen-independent. These results suggest that antigen-activated T cells express covert interaction sites for macrophages and that these sites are actively covered-up by other membrane components.

T lymphocyte recognition of peptide antigens: evidence favoring the formation of neoantigenic determinants

To more precisely define the nature of exogenous antigenic determinants recognized by T cells, the response to fibrinopeptide fragment B beta 7-14 and a peptide of the inverted amino acid sequence of B beta 7-14 was examined. Strain 2 guinea pig T cells immunized with B beta 7-14 showed in vitro proliferative responses with B beta 7-14, but failed to respond to the inverted B beta 7-14 peptide. Moreover, the inverted B beta 7-14 peptide was nonimmunogenic and failed to prime strain 2 T cells for responses to native or inverted B beta 7-14. These results suggest that T cell recognition of peptide antigens involves more than simple interactions with amino acid side chains and that the ordering of the amino acids within the peptide is critical. One interpretation of these results is that T cells exhibit polarity in reading of antigenic determinants and peptides become associated with self in some fashion to form a neoantigenic determinant. To test this possibility, a Gly residue was added to the carboxyl end of B beta 7-14 (B beta 7-15), which is the likely site of attachment to self. It was found that strain 13 guinea pigs, which are totally unresponsive to B beta 7-14, produced T cell responses to B beta 7-15. This observation is consistent with the interpretation that Gly spaces the B beta 7-14 away from self to form an antigenic determinant complementary to strain 13 T cell antigen recognition structures. These results are discussed with respect to several mechanisms for immune response gene control of T cell responses.