The immunogenic capacity of antigen taken up by peritoneal exudate cells

The immunogenic capacity of protein antigens has been compared for the free and peritoneal exudate cell (PEC)-bound forms. The response of CBA mice to BSA provides the reference system, but lysozyme, ovalbumin, HSA and modified BSA were also studied. Uptake is approximately equally efficient in vivo and in vitro. PEC-bound antigen, estimated by radioactivity is far more potent than the free form in inducing primary immunization. The following properties were also found: (i) viable PEC are required; (ii) irradiation of the cell donor 2–7 days before giving antigen inhibits immunization, but irradiation after uptake does not do so; (iii) mice are susceptible to immunization during their phase of recovery from paralysis; (iv) PEC do not retain large amounts of antigen for long; (v) the activity does not depend solely on a minor, phagocytosisprone fraction of the antigen; (vi) allogeneic transfer of PEC reduces their immunogenic capacity; (vii) paralysed hosts are not susceptible to immunization, but PEC from paralysed donors are effective; and (viii) the enhancement of immunogenic capacity does not apply to the secondary response. The conclusion may be drawn that the retention of small quantities of antigen by macrophages plays an essential role in some, but probably not all types of immune response.

The subcellular distribution of antigen in macrophages

The intracellular fate of phagocytosed antigens in cells from peritoneal exudate in CBA mice has been studied by using (126)I and (131)I labeled antigens. After uptake of labeled antigen, cells were homogenized and the subcellular fractions were analyzed by isopycnic centrifugation in a sucrose gradient. The uptake of heat-denatured BSA (c BSA) by these cells in vivo is 3.5 microg/mg c BSA injected/10(8) cells. The uptake by cells in animals which were exposed 2 days earlier to 900 r whole body irradiation is slightly lower but does not differ significantly. 90% of the phagocytosed material is degraded within 2-3 hr, the residual 10% is retained at least over an 8 hr periods. Using a pulse and chase technique, with (125)I and (131)I c BSA in vitro and in vivo it was shown that newly phagocytosed antigen is found mainly in a lysosomal turnover compartment of a density 1.19 g cm(-3). Antigen which has been in the cells for longer was found in a denser fraction (1.26 g cm(-3)). In a comparison of nhrmal and X-irradiated cells it can be shown that after irradiation with 900 r less c BSA is found in this storage compartment. Binding of the antigen to the subcellular fractions, and its behavior towards several detergents has been studied. Subcellular fractions do not have the increased immunogenic capacity of antigen enclosed in living macrophages. Two synthetic polypeptide antigens, poly(D-Tyr, D-Glu, D-Ala) and poly-(L-Tyr, L-Glu) have a different subcellular distribution from c BSA, BSA, or bovine gamma globulin. Apart from also being found in the 1.26 storage compartment the polypeptide antigens are mainly located in a 1.15 compartment and only to a small extent in the 1.19 compartment. The half-life of these antigens in the cells is much longer than the half-life of the protein antigens studied. The finding of several subcellular compartments is discussed in connection with the functions possibly performed by macrophages.

Immunological paralysis induced by brief exposure of cells to protein antigens

The paralysing effect of brief exposure to antigen has been studied in a system in which cells were exposed and then tested by challenge immunization after transfer into syngeneic unresponsive mice. Bovine and human serum albumins (BSA and HSA) were used chiefly, but lysozyme was also tested. Irradiation and paralysis on their own had drawbacks as methods of procuring unresponsive hosts, and worked best in combination. The response measured in the hosts as antigen binding capacity (ABC) was found to be proportional to the number of reactive cells transferred. Lymph node cells and blood leucocytes proved more active than spleen cells. Cells did not become paralysed immediately upon exposure to high concentrations of antigen; blood leucocytes required approximately 2 hours in vivo exposure, and cells from lymphoid organs approximately 24 hours. The effectiveness of 24 hours exposure depended on antigen dose. Tentative measurements of the time taken for antigen to diffuse through the organs suggested that this delay would not account for the difference in timing between cells of different anatomical origin. Exposure in vitro was not found repeatably effective in inducing paralysis. The conclusion was drawn that paralysis is induced by the direct action of antigen on lymphocytes, but that the cells may undergo some further step of differentiation before induction is complete.

The dosage requirements for immunological paralysis by soluble proteins

The quantitative dose requirements for induction of paralysis by BSA in mice has been the subject of further study. Parallel studies have been made with lysozyme, ovalbumin, diphtheria toxoid and ribonuclease, in which similar paralysing and immunizing procedures were used, and similar direct binding tests applied to measurement of the response. In normal adults all the antigens tested induced high-zone paralysis and concomitant immunization, but BSA alone induced low-zone paralysis. With irradiation, with courses of injection commencing at birth, and with paralysis-maintaining treatment, all the antigens tested induced paralysis in a zone quantitatively similar to the low zone detectable in normal adults with BSA. Neither irradiation, treatment with cortisol, nor thymectomy affected the rate of induction of paralysis in the low zone. On the other hand the minimum dose required for immunization varied markedly from one antigen to another.

The ability of BSA to induce low-zone paralysis in normal adults can, therefore, be attributed to the failure of low doses of this antigen to immunize. The consistency of paralysis threshold, in contrast to the variability for immunization, is interpreted as evidence of an additional step of complexity involved in immunization that is not required for paralysis.

Chemical and serological studies with an iodine-containing synthetic immunological determinant 4-hydroxy-3-iodo-5-nitrophenylacetic acid (NIP) and related compounds

The synthesis and properties of 4-hydroxy-3-iodo-5-nitrophenylacetic acid (NIP) and several related compounds are described. Conjugates of NIP with proteins are prepared from the azide, synthesized from commercial 4-hydroxy-phenylacetic acid. Sera from rabbits and mice immunized with NIP—ovalbumin or NIP—chicken serum globulin bind N¹³¹IP-containing compounds, as judged from precipitation of radioactivity by salting-out of immunoglobulins. Homogeneous binding is obtained with N¹³¹IP—polylysine, N¹³¹IP—ε-amino-n-caproic acid (N¹³¹IP—aminocap), and other structurally related haptens; non-homogeneous binding is obtained with N¹³¹IP—bovine serum albumin. Binding to salt-precipitated immunoglobulin of N¹³¹IP—aminocap, the hapten of choice for this purpose, provides an assay for antibody measurable at concentrations down to at least M^-9 serum binding capacity (∼0.1 μg antibody/ml). Structurally related compounds and NIP—protein conjugates competitively inhibit binding of N¹³¹IP—aminocap. The inhibitions indicate that the iodine contained in NIP, but not the carrier protein, contributes significantly to the binding site.

Recovery from immunological paralysis in relation to age and residual antigen

Mice paralysed by bovine serum albumin gradually recover immunological reactivity. Recovery has been mapped at various ages by challenge with antigen in Freund's adjuvant, and comparing the resulting antigen binding capacity of the serum with that of normal controls. Recovery proceeds more rapidly in young than in old animals, irrespective of the duration of previous exposure to antigen. This is considered to confirm the importance of cell turnover in recovery. In harmony with this hypothesis, irradiation proved capable of deleting an existing state of paralysis without appreciably enhancing recovery. A cell transfer test has been used to detect antigen in paralysed animals, supplemented by measurement of radioactivity from [¹³¹I]labelled antigen. Antigen can be detected throughout the lag period preceding the start of recovery in quantities probably sufficient to account for the lag.

The role of cell number and source in adoptive immunity

The use of bacteriophage and bovine serum albumin as antigens for adoptive immunization has been investigated. Cells were transplanted from normal or previously immunized (`primed cells') donor mice into isologous hosts, and then stimulated with antigen. The reactions to phage of normal and primed cells could be distinguished and measured; for BSA, only primed cells could be used without the help of adjuvant. Irradiation of the prospective host enhanced the activity of the transplanted cells. Saturation was easily reached by increasing the number of phage-primed cells in the transplant. BSA-primed cells saturated less easily, probably because BSA is a relatively poor immunogen. Primed cells were localized, to a limited extent, in the lymph nodes draining the sites of injection of antigen, but could usually also be found in the blood.

The effect of antigen dosage on the response of adoptively transferred cells

Mice were immunized with BSA or HSA in Freund's adjuvant, and their lymph node and spleen cells transplanted into syngeneic hosts, which in most experiments had been irradiated. After transplantation the cells do not synthesize much antibody if left without stimulation, but can be stimulated to do so by injection of BSA or HSA in solution. The response has been studied over a dose range of 10^-3–10⁵ μg. antigen. Stimulation can be detected down to 10^-3 μg. antigen, and reaches a maximum at middling doses. Middling doses stimulate proliferation of the primed cells to an extent which can be measured by ¹³¹IUdR uptake. At high doses both antibody production and IUdR uptake are inhibited. The conclusion is drawn that high concentrations of antigen can paralyse the immunological reaction of primed cells.