Studies on the capacity of intact cells and purified Ia from different B cell sources to function in antigen presentation to T cells

In this study we have evaluated some of the potential mechanisms that may be responsible for the inefficiency with which resting B cells function as antigen-presenting cells (APC) and the mechanism by which that function is enhanced following treatment of B cells with neuraminidase. One mechanism that has been previously suggested is that glycosylation differences in Ia associated with different APC accounts for the different functional capacities of resting and activated B cells. It has been postulated that removal of sialic acid from resting B cell Ia results in a correction of its antigen-presenting defect. To study this possibility, we have used purified I-Ad from different B cell sources in a planar membrane system to present an immunogenic peptide of chicken ovalbumin (Ova) to an I-Ad-restricted Ova-specific T cell hybridoma. It was found that I-Ad isolated from resting B cells, B cell stimulatory factor 1 (BSF-1) or lipopolysaccharide and dextran sulfate-stimulated B cells, or A20 B lymphoma cells were all equivalent in their antigen-presenting capacity. Furthermore, removal of sialic acid from Ia did not enhance its capacity to serve as a restriction element. The mechanism by which neuraminidase treatment enhances B cell APC function was further investigated by studying the effect of sialic acid removal on a primary mixed leukocyte reaction (MLR). When allogeneic fixed B cells were used as stimulator cells it was found that neither resting nor BSF-1-stimulated B cells could induce a MLR. Following neuraminidase treatment, BSF-1-treated B cells, but not resting B cells, were capable of stimulating a MLR. However, a MLR was also stimulated by allogeneic BSF-1-treated B cells when the responder T cells, rather than the stimulator cells, were treated with neuraminidase. An enhancing effect similar to that obtained by neuraminidase treatment could be obtained by the addition of 2% polyethylene glycol to the MLR culture. These data suggest that the inability of BSF-1-stimulated cells to function efficiently as accessory cells in stimulating a primary MLR is due to their relative inability to interact physically with T cells, a deficiency that is overcome by neuraminidase treatment of either T or B cell populations or by the addition of polyethylene glycol to the culture. Although the reason for the failure of these same treatments to restore the accessory cell function of resting B cells is not known, some possible mechanisms are discussed.

Studies on the mechanism of stimulation of T cells by the Mycoplasma arthritidis-derived mitogen. Role of class II IE molecules

A mitogen derived from the supernatant of broth cultures of Mycoplasma arthritidis (MAS-P) stimulates a proliferative response by normal, unprimed T cells and interleukin 2 production by some, but not all, T cell hybridomas. The response requires an IE-positive accessory cell (AC). The direct participation of IE, and not IA, in this system was confirmed by two sets of experiments. First, L cells transfected with IE, but not IA, provided effective AC function for both normal T cells and the T cell hybridoma DO-11.10. Second, we have taken a more direct approach by showing that purified IE incorporated in liposomes and used to coat glass beads can support the MAS-P response of the DO-11.10 T cell hybridoma in the absence of intact AC or other AC molecules. Although the receptor for IE-MAS-P has not been identified, we have eliminated from consideration two potential T cell recognition structures. Monoclonal antibody to the antigen-major histocompatibility complex specific receptor failed to inhibit the MAS-P response of DO-11.10 or the T cell line LBRM-33. Furthermore, the L3T4 molecule did not appear to be involved since an L3T4-negative variant of DO-11.10 responded well to the mitogen. In addition, we show that both Lyt-2-positive and L3T4-positive T cells respond to this class II-restricted stimulus. Thus, we postulate the existence of a non-T cell receptor, non-L3T4 receptor that recognizes MAS-P in association with a presumed nonpolymorphic region of IE.

Studies on the mechanism of stimulation of T cells by the Mycoplasma arthritidis-derived mitogen. Role of class II IE molecules

A mitogen derived from the supernatant of broth cultures of Mycoplasma arthritidis (MAS-P) stimulates a proliferative response by normal, unprimed T cells and interleukin 2 production by some, but not all, T cell hybridomas. The response requires an IE-positive accessory cell (AC). The direct participation of IE, and not IA, in this system was confirmed by two sets of experiments. First, L cells transfected with IE, but not IA, provided effective AC function for both normal T cells and the T cell hybridoma DO-11.10. Second, we have taken a more direct approach by showing that purified IE incorporated in liposomes and used to coat glass beads can support the MAS-P response of the DO-11.10 T cell hybridoma in the absence of intact AC or other AC molecules. Although the receptor for IE-MAS-P has not been identified, we have eliminated from consideration two potential T cell recognition structures. Monoclonal antibody to the antigen-major histocompatibility complex specific receptor failed to inhibit the MAS-P response of DO-11.10 or the T cell line LBRM-33. Furthermore, the L3T4 molecule did not appear to be involved since an L3T4-negative variant of DO-11.10 responded well to the mitogen. In addition, we show that both Lyt-2-positive and L3T4-positive T cells respond to this class II-restricted stimulus. Thus, we postulate the existence of a non-T cell receptor, non-L3T4 receptor that recognizes MAS-P in association with a presumed nonpolymorphic region of IE.

Structural characteristics of an antigen required for its interaction with Ia and recognition by T cells

A detailed analysis of the residues within an immunogenic peptide that endow it with the capacity to interact with Ia and to be recognized by T cells is presented. Ia interacts with only a few of the peptide residues and overall exhibits a very broad specificity. Some residues appear to interact both with Ia and with T cells, leading to a model in which a peptide antigen is 'sandwiched' between Ia and the T-cell receptor.

The relation between major histocompatibility complex (MHC) restriction and the capacity of Ia to bind immunogenic peptides

The capacity of purified I-Ad, I-Ed, I-Ak, and I-Ek to bind to protein derived peptides that have been previously reported to be T cell immunogens has been examined. For each of the 12 peptides studied strong binding to the relevant Ia restriction element was observed. All the peptides bound more than one Ia molecule; however, for 11 of 12 peptides, the dominant binding was to the restriction element, whereas in one instance the dominant binding was to a nonrestriction element. When the peptides were used to inhibit the presentation of antigen by prefixed accessory cells to T cells, an excellent correlation was found between the capacity of a peptide to inhibit the binding of an antigen to purified Ia and the capacity of the peptide to inhibit accessory cell presentation of the antigen. Thus, the binding of peptide to purified Ia is immunologically relevant, and Ia seems to be the only saturable molecule on the surface of the accessory cell involved in antigen presentation. Inhibition analysis also indicated that all peptides restricted to a particular Ia molecule competitively inhibited one another, suggesting that each Ia restriction element has a single binding site for antigen. Cross-linking of labeled peptides to Ia followed by electrophoretic analysis and autoradiography suggested that this single binding site is made up of portions of both alpha and beta chains of Ia.

Immunological self, nonself discrimination

The ability of immunodominant peptides derived from several antigen systems to compete with each other for T cell activation was studied. Only peptides restricted by a given transplantation antigen are mutually competitive. There is a correlation between haplotype restriction, ability to bind to the appropriate transplantation antigen, and ability to inhibit activation of other T cells restricted by the same transplantation antigen. An exception was noted in that a peptide derived from an antigen, bacteriophage lambda cI repressor, binds to the I-Ed molecule in a specific way, yet is not I-Ed-restricted. Comparison of the sequence of the repressor peptide with that of other peptides able to bind to (and be restricted by) I-Ed and a polymorphic region of the I-Ed molecule itself revealed a significant degree of homology. Thus, peptides restricted by a given class II molecule appear to be homologous to a portion of the class II molecule itself. The repressor-derived peptide is identical at several polymorphic residues at this site, and this may account for the failure of I-Ed to act as a restriction element. Comparison of antigenic peptide sequences with transplantation antigen sequences suggests a model that provides a basis for explaining self, nonself discrimination as well as alloreactivity.

Isolation and characterization of antigen-Ia complexes involved in T cell recognition

Using equilibrium dialysis, it has been previously demonstrated that immunogenic peptides bind specifically to the Ia molecules serving as restriction elements in the immune response to these antigens. Using gel filtration to study the formation of ovalbumin (OVA) peptide-I-Ad complexes, it is herein demonstrated that the complexes, once formed, are very stable (kd approximately equal to 3 X 10(-6) s-1), but the rate of complex formation is very slow (ka approximately 1 M-1 s-1 explaining the overall low equilibrium constant of approximately 2 X 10(-6) M. Treating the complexes with glutaraldehyde revealed that the ovalbumin peptide was cross-linked solely to the alpha chain of I-Ad. Planar membranes containing I-Ad-OVA complexes stimulated a T cell response with 2 X 10(4) less antigen than required when uncomplexed antigen was used, thus demonstrating the biologic importance of these complexes in antigen recognition.

Capacity of B cells to function as stimulators of a primary mixed leukocyte reaction

The capacity of B cells to serve as stimulator cells for a primary mixed leukocyte reaction (MLR) was evaluated. Percoll-fractionated B cells were stimulated with lipopolysaccharide and dextran sulfate (L/D) or a B cell stimulatory factor (BSF-1)-containing culture supernatant, and then were fixed before being used as stimulator cells to more precisely define the state of activation associated with MLR stimulatory capacity. It was found that unstimulated B cells or B cells stimulated for 1 day with L/D or BSF-1 were incapable of initiating a primary MLR, whereas B cells incubated for 3 days in L/D were potent stimulators. The differential activity of 1 day L/D- and BSF-1-activated B cells compared with 3 day L/D-activated B cells was not related to the amount of the relevant MHC class I or class II alloantigens on these cell populations, because all three groups had large increments in MHC class II expression in the following order: BSF-1 greater than 3 day L/D greater than 1 day L/D, and had little difference in MHC class I expression. Also, all three populations were capable of stimulating both MHC class I- and class II-specific T cell hybrids. It was concluded that the capacity of 3 day L/D-activated cells to stimulate a primary MLR was due to the elaboration of necessary co-stimulator molecules. We evaluated whether interleukin 1 (IL 1) was the co-stimulator involved. That this was not the case was indicated by two findings. First, 3 day-activated L/D cells failed to express IL 1 activity as measured by a highly sensitive IL 1 assay that utilizes the T cell line D10.G4.1. Second, recombinant IL 1 added to MLR cultures containing 1 day L/D- or BSF-1 activated B cells failed to function as a co-stimulator. In contrast, the phorbol ester PMA was a potent co-stimulator in this system. We conclude from these experiments that appropriately activated B cells can function as stimulators of a primary MLR, and that they elaborate critical co-stimulator molecules, distinct from IL 1, that enable them to function in this regard.

Capacity of B cells to function as stimulators of a primary mixed leukocyte reaction

The capacity of B cells to serve as stimulator cells for a primary mixed leukocyte reaction (MLR) was evaluated. Percoll-fractionated B cells were stimulated with lipopolysaccharide and dextran sulfate (L/D) or a B cell stimulatory factor (BSF-1)-containing culture supernatant, and then were fixed before being used as stimulator cells to more precisely define the state of activation associated with MLR stimulatory capacity. It was found that unstimulated B cells or B cells stimulated for 1 day with L/D or BSF-1 were incapable of initiating a primary MLR, whereas B cells incubated for 3 days in L/D were potent stimulators. The differential activity of 1 day L/D- and BSF-1-activated B cells compared with 3 day L/D-activated B cells was not related to the amount of the relevant MHC class I or class II alloantigens on these cell populations, because all three groups had large increments in MHC class II expression in the following order: BSF-1 greater than 3 day L/D greater than 1 day L/D, and had little difference in MHC class I expression. Also, all three populations were capable of stimulating both MHC class I- and class II-specific T cell hybrids. It was concluded that the capacity of 3 day L/D-activated cells to stimulate a primary MLR was due to the elaboration of necessary co-stimulator molecules. We evaluated whether interleukin 1 (IL 1) was the co-stimulator involved. That this was not the case was indicated by two findings. First, 3 day-activated L/D cells failed to express IL 1 activity as measured by a highly sensitive IL 1 assay that utilizes the T cell line D10.G4.1. Second, recombinant IL 1 added to MLR cultures containing 1 day L/D- or BSF-1 activated B cells failed to function as a co-stimulator. In contrast, the phorbol ester PMA was a potent co-stimulator in this system. We conclude from these experiments that appropriately activated B cells can function as stimulators of a primary MLR, and that they elaborate critical co-stimulator molecules, distinct from IL 1, that enable them to function in this regard.

Activation requirements for normal T cells: accessory cell-dependent and -independent stimulation by anti-receptor antibodies

We have examined the requirements for the activation of normal T cells by two anti-T cell receptor antibody preparations, including a rabbit antiserum, R3497, which binds to all normal T cells, and a rat monoclonal antibody, KJ16-133, which binds to about 20% of T cells. The requirements for stimulation of T cells by both antibodies were similar. Soluble antibodies in the absence of accessory cells (AC) failed to induce either proliferation or the expression of IL 2 receptors, and the addition of either IL 2 or PMA failed to synergize with these soluble antibodies for an AC-independent proliferative response. Activation could only be achieved in the presence of Fc receptor-positive AC, although Fc receptor expression alone appeared not to be sufficient for AC activity because some Fc receptor-positive cells did not function in this capacity. Activation with anti-receptor antibody conjugated to Sepharose 4B beads could be demonstrated in the presence of some exogenous cofactors, such as IL 2 and PMA, but not in the presence of recombinant IL 1. When activation by soluble antibody plus AC was compared to activation by bead-conjugated antibody + recombinant IL 2, it was found that the former favored the stimulation of Lyt-2+ cells. The effects of the addition of anti-L3T4 monoclonal antibody was also examined in this system. Anti-L3T4 inhibited the response of L3T4+ cells when used in the presence of Ia+ as well as Ia- AC, and it also inhibited activation in a system in which KJ16-133 conjugated to Sepharose was used in the absence of AC. Because anti-L3T4 had an inhibitory effect in the presence of Ia- AC as well as in the absence of any AC, it is concluded that L3T4 does not necessarily function by interacting with Ia on the surface of AC, and may directly transmit down-regulatory signals when bound by anti-L3T4.

Activation requirements for normal T cells: accessory cell-dependent and -independent stimulation by anti-receptor antibodies

We have examined the requirements for the activation of normal T cells by two anti-T cell receptor antibody preparations, including a rabbit antiserum, R3497, which binds to all normal T cells, and a rat monoclonal antibody, KJ16-133, which binds to about 20% of T cells. The requirements for stimulation of T cells by both antibodies were similar. Soluble antibodies in the absence of accessory cells (AC) failed to induce either proliferation or the expression of IL 2 receptors, and the addition of either IL 2 or PMA failed to synergize with these soluble antibodies for an AC-independent proliferative response. Activation could only be achieved in the presence of Fc receptor-positive AC, although Fc receptor expression alone appeared not to be sufficient for AC activity because some Fc receptor-positive cells did not function in this capacity. Activation with anti-receptor antibody conjugated to Sepharose 4B beads could be demonstrated in the presence of some exogenous cofactors, such as IL 2 and PMA, but not in the presence of recombinant IL 1. When activation by soluble antibody plus AC was compared to activation by bead-conjugated antibody + recombinant IL 2, it was found that the former favored the stimulation of Lyt-2+ cells. The effects of the addition of anti-L3T4 monoclonal antibody was also examined in this system. Anti-L3T4 inhibited the response of L3T4+ cells when used in the presence of Ia+ as well as Ia- AC, and it also inhibited activation in a system in which KJ16-133 conjugated to Sepharose was used in the absence of AC. Because anti-L3T4 had an inhibitory effect in the presence of Ia- AC as well as in the absence of any AC, it is concluded that L3T4 does not necessarily function by interacting with Ia on the surface of AC, and may directly transmit down-regulatory signals when bound by anti-L3T4.

Interaction between a "processed" ovalbumin peptide and Ia molecules

The binding of 125I-labeled immunogenic peptides to purified Ia molecules in detergent solution was examined by equilibrium dialysis. We used the chicken ovalbumin peptide ovalbumin-(323-339)-Tyr, which is immunogenic in the BALB/c mouse and restricted to I-Ad. 125I-labeled ovalbumin-(323-339)-Tyr was shown to bind to I-Ad but not to I-Ed, I-Ek, or I-Ak. This binding was inhibited by unlabeled ovalbumin-(323-339) but not by ovalbumin-(329-339), which is the longest N-terminally truncated peptide that fails to stimulate any of the I-Ad-restricted hybridomas that have been raised to ovalbumin-(323-339)-Tyr. As a further specificity control, we also used the chicken egg lysozyme peptide Tyr-(46-61), which has recently been studied by similar methods [Babbitt, B. P., Allen, P. M., Matsueda, G., Haber, E. & Unanue, E. R. (1985) Nature (London) 317, 359-361]. We have confirmed that it bound to I-Ak but not to I-Ek, I-Ad, or I-Ed. Thus, a specific interaction between Ia and antigen that correlates with the major histocompatibility complex restriction was demonstrated, strongly arguing in favor of a determinant selection hypothesis for such restriction.

Ia-specific mixed leukocyte reactive T cell hybridomas: analysis of their specificity by using purified class II MHC molecules in synthetic membrane system

This study was undertaken to determine the nature of the antigens recognized in allogeneic and syngeneic mixed leukocyte reactions (MLR). Specifically, we wished to determine whether Ia antigens alone were recognized by MLR-reactive T cells, or whether the specificity was determined by the corecognition of non-MHC antigens together with syngeneic or allogeneic Ia. To do this we used 11 T cell hybrids that were characterized as being specific for Iad and were tested their capacity to respond to isolated I-Ad or I-Ed that had been incorporated into liposomes and had bound to the surface of glass beads. Of nine alloreactive T cell hybrids (five I-Ad-and four I-Ed-specific), seven were shown to be responsive to the relevant isolated Ia antigen on glass beads. Also, two of two syngeneic I-Ad-specific T cell hybrids responded to I-Ad on the glass beads. One of the two alloreactive T cell hybrids that failed to respond to the relevant Ia antigen on glass beads was shown to be specific for an antigen in fetal calf serum (FCS) that was recognized in the context of the allo-Ia antigen (I-Ed), because when intact accessory cells were used, a response by this hybrid was only observed when FCS was present in the assay culture medium or when the accessory cells were pre-pulsed with FCS. The possible involvement of FCS antigens and non-Ia accessory cell antigens in the stimulation of the nine T cell hybrids that responded to isolated Ia on glass beads was evaluated. T cell hybrids that were grown and were tested in serum free medium were still capable of reacting to Ia on beads. The isolated Ia preparations used were greater than 90% pure, and their capacity to stimulate the T cell hybrids did not correlate with the degree of contamination with non-Ia proteins. We conclude from these studies that the majority of T cells that respond to allogeneic or syngeneic Ia bearing stimulator cells are specific for the Ia antigens themselves, and do not require the co-recognition of other non-Ia antigens; nor is there any requirement for Ia antigen processing for this recognition.

Ia-specific mixed leukocyte reactive T cell hybridomas: analysis of their specificity by using purified class II MHC molecules in synthetic membrane system

This study was undertaken to determine the nature of the antigens recognized in allogeneic and syngeneic mixed leukocyte reactions (MLR). Specifically, we wished to determine whether Ia antigens alone were recognized by MLR-reactive T cells, or whether the specificity was determined by the corecognition of non-MHC antigens together with syngeneic or allogeneic Ia. To do this we used 11 T cell hybrids that were characterized as being specific for Iad and were tested their capacity to respond to isolated I-Ad or I-Ed that had been incorporated into liposomes and had bound to the surface of glass beads. Of nine alloreactive T cell hybrids (five I-Ad-and four I-Ed-specific), seven were shown to be responsive to the relevant isolated Ia antigen on glass beads. Also, two of two syngeneic I-Ad-specific T cell hybrids responded to I-Ad on the glass beads. One of the two alloreactive T cell hybrids that failed to respond to the relevant Ia antigen on glass beads was shown to be specific for an antigen in fetal calf serum (FCS) that was recognized in the context of the allo-Ia antigen (I-Ed), because when intact accessory cells were used, a response by this hybrid was only observed when FCS was present in the assay culture medium or when the accessory cells were pre-pulsed with FCS. The possible involvement of FCS antigens and non-Ia accessory cell antigens in the stimulation of the nine T cell hybrids that responded to isolated Ia on glass beads was evaluated. T cell hybrids that were grown and were tested in serum free medium were still capable of reacting to Ia on beads. The isolated Ia preparations used were greater than 90% pure, and their capacity to stimulate the T cell hybrids did not correlate with the degree of contamination with non-Ia proteins. We conclude from these studies that the majority of T cells that respond to allogeneic or syngeneic Ia bearing stimulator cells are specific for the Ia antigens themselves, and do not require the co-recognition of other non-Ia antigens; nor is there any requirement for Ia antigen processing for this recognition.

Antigen presentation by splenic B cells: resting B cells are ineffective, whereas activated B cells are effective accessory cells for T cell responses

In this study, we have investigated the ability of splenic B cells to act as antigen-presenting cells. Previous data had established that lipopolysaccharide (LPS)-activated B cells were effective antigen-presenting cells; however, the relative capacity of resting B cells to carry out this function remains controversial. Splenic B cells from naive BALB/c mice were depleted of macrophages, dendritic cells, and T cells, and were fractionated on the basis of cell density by using Percoll gradient centrifugation. Fractions were collected from the 50/60, 60/65, and 65/72% interfaces and from greater than 72% (pellet). Cytofluorograph analysis of the fractionated B cells showed that the two lower density fractions (50/60 and 60/65) contained a number of cells which, by cell size determination, appeared to be activated B cells, whereas the two higher density fractions (65/72 and greater than 72) appeared to contain predominantly small resting B cells contaminated by many fewer activated B cells. Functionally, the capacity of fractionated B cells to act as accessory cells for a concanavalin A response or present the antigens chicken ovalbumin (OVA) or OVA-tryptic digest gave similar results, which indicated a striking hierarchy of accessory cell function in the different Percoll fractions. When normalized to the most active low-density fraction (50/60%), the activity of the other fractions were: 60/65 = 78%; 65/72 = 25%; and greater than 72 = 4%. The differences in the functional capacity between the various Percoll fractions did not appear to be due to differences in Ia expression. Although the expression of Ia varied approximately 12-fold within any one fraction, there was little difference in the mean amount of Ia on cells obtained from the various fractions. Kinetic studies showed that activation of B cells with LPS and dextran sulfate resulted in the expression of two stages of functional development. The first stage was an increased efficiency of accessory cell function that was abrogated by irradiation with 4000 rad followed by a second stage, which was characterized by the acquisition of resistance to treatment with 4000 rad. When nonfractionated B cells that had been stimulated with LPS and DexSO4 were sorted on the basis of cell size into a small B cell fraction and a large B cell fraction, only the large B cells were able to present antigen. Taken together, these data suggest that much of the accessory cell function associated with splenic B cells can be accounted for by the relatively small percentage of activated B cells present in the spleen.(ABSTRACT TRUNCATED AT 400 WORDS)

Antigen presentation by splenic B cells: resting B cells are ineffective, whereas activated B cells are effective accessory cells for T cell responses

In this study, we have investigated the ability of splenic B cells to act as antigen-presenting cells. Previous data had established that lipopolysaccharide (LPS)-activated B cells were effective antigen-presenting cells; however, the relative capacity of resting B cells to carry out this function remains controversial. Splenic B cells from naive BALB/c mice were depleted of macrophages, dendritic cells, and T cells, and were fractionated on the basis of cell density by using Percoll gradient centrifugation. Fractions were collected from the 50/60, 60/65, and 65/72% interfaces and from greater than 72% (pellet). Cytofluorograph analysis of the fractionated B cells showed that the two lower density fractions (50/60 and 60/65) contained a number of cells which, by cell size determination, appeared to be activated B cells, whereas the two higher density fractions (65/72 and greater than 72) appeared to contain predominantly small resting B cells contaminated by many fewer activated B cells. Functionally, the capacity of fractionated B cells to act as accessory cells for a concanavalin A response or present the antigens chicken ovalbumin (OVA) or OVA-tryptic digest gave similar results, which indicated a striking hierarchy of accessory cell function in the different Percoll fractions. When normalized to the most active low-density fraction (50/60%), the activity of the other fractions were: 60/65 = 78%; 65/72 = 25%; and greater than 72 = 4%. The differences in the functional capacity between the various Percoll fractions did not appear to be due to differences in Ia expression. Although the expression of Ia varied approximately 12-fold within any one fraction, there was little difference in the mean amount of Ia on cells obtained from the various fractions. Kinetic studies showed that activation of B cells with LPS and dextran sulfate resulted in the expression of two stages of functional development. The first stage was an increased efficiency of accessory cell function that was abrogated by irradiation with 4000 rad followed by a second stage, which was characterized by the acquisition of resistance to treatment with 4000 rad. When nonfractionated B cells that had been stimulated with LPS and DexSO4 were sorted on the basis of cell size into a small B cell fraction and a large B cell fraction, only the large B cells were able to present antigen. Taken together, these data suggest that much of the accessory cell function associated with splenic B cells can be accounted for by the relatively small percentage of activated B cells present in the spleen.(ABSTRACT TRUNCATED AT 400 WORDS)

Transfer of antigen-presenting capacity to Ia-negative cells upon fusion with Ia-bearing liposomes

The role of Ia in T cell activation was investigated by incorporating affinity-purified I-Ad molecules into synthetic liposomal membranes and by using these as antigen-presenting units. IL 2 production by I-Ad-restricted, chicken ovalbumin-specific T cell hybridomas was measured in a system in which antigen processing by the presenter was not required. I-Ad-bearing liposomes were found to have no antigen-presenting capacity. It was shown, however, that antigen-presenting capacity could be conferred on Ia-negative cells by fusion of these cells with liposomes bearing I-Ad molecules, together with Sendai virus envelope glycoproteins, as fusogenic agents. Both Ia-negative B lymphoma cells and mouse L cells were capable of antigen presentation of predigested ovalbumin after fusion with vesicles formed from phosphatidylserine and phosphatidylethanolamine in a 1:1 w:w ratio. The cell surface expression of the transferred Ia remained stable for at least 7 hr. These results indicate that Ia is the only additional cell surface molecule required, at least by Ia-negative B cell lymphomas and L cells, to convert them into effective antigen-presenting cells. This system should be useful in future studies of the cellular requirements for antigen processing and presentation.

Transfer of antigen-presenting capacity to Ia-negative cells upon fusion with Ia-bearing liposomes

The role of Ia in T cell activation was investigated by incorporating affinity-purified I-Ad molecules into synthetic liposomal membranes and by using these as antigen-presenting units. IL 2 production by I-Ad-restricted, chicken ovalbumin-specific T cell hybridomas was measured in a system in which antigen processing by the presenter was not required. I-Ad-bearing liposomes were found to have no antigen-presenting capacity. It was shown, however, that antigen-presenting capacity could be conferred on Ia-negative cells by fusion of these cells with liposomes bearing I-Ad molecules, together with Sendai virus envelope glycoproteins, as fusogenic agents. Both Ia-negative B lymphoma cells and mouse L cells were capable of antigen presentation of predigested ovalbumin after fusion with vesicles formed from phosphatidylserine and phosphatidylethanolamine in a 1:1 w:w ratio. The cell surface expression of the transferred Ia remained stable for at least 7 hr. These results indicate that Ia is the only additional cell surface molecule required, at least by Ia-negative B cell lymphomas and L cells, to convert them into effective antigen-presenting cells. This system should be useful in future studies of the cellular requirements for antigen processing and presentation.

Accessory cell function in the Con A response: role of Ia-positive and Ia-negative accessory cells

We have examined the role of Ia-positive and Ia-negative accessory cells (AC) and soluble factors in Con A-stimulated murine T cell activation. Supernatant fluids containing interleukin 1 (IL 1) derived from the P388D1 macrophage cell line and from a lipopolysaccharide (LPS)-stimulated macrophage hybridoma provided only partial reconstitution of the response of purified T cells (18 to 27%). The complete reconstitution obtained with gamma-irradiated spleen cells or LPS-activated B cells was inhibited by approximately 60 to 77% when anti-Ia antibody was included in the culture. Despite this apparent involvement of Ia+ spleen AC, Ia-negative L cell AC could also reconstitute the response of both Class I-restricted Lyt-2+ T cells and Class II-restricted L3T4+ T cells. When the Ia-negative AC were employed, the L3T4 antigen on L3T4+ T cells played a critical role because addition of anti-L3T4 antibody to the culture inhibited the response by 85 to 90%. In contrast, anti-L3T4 did not inhibit the response in the presence of spleen AC. These results suggest that the molecules involved in T cell-AC interactions may vary depending on the AC source. Moreover, at least one of the putative target ligands for L3T4 presumably is not Ia, because anti-L3T4 inhibited T cell stimulation when Ia-negative AC were used.

Accessory cell function in the Con A response: role of Ia-positive and Ia-negative accessory cells

We have examined the role of Ia-positive and Ia-negative accessory cells (AC) and soluble factors in Con A-stimulated murine T cell activation. Supernatant fluids containing interleukin 1 (IL 1) derived from the P388D1 macrophage cell line and from a lipopolysaccharide (LPS)-stimulated macrophage hybridoma provided only partial reconstitution of the response of purified T cells (18 to 27%). The complete reconstitution obtained with gamma-irradiated spleen cells or LPS-activated B cells was inhibited by approximately 60 to 77% when anti-Ia antibody was included in the culture. Despite this apparent involvement of Ia+ spleen AC, Ia-negative L cell AC could also reconstitute the response of both Class I-restricted Lyt-2+ T cells and Class II-restricted L3T4+ T cells. When the Ia-negative AC were employed, the L3T4 antigen on L3T4+ T cells played a critical role because addition of anti-L3T4 antibody to the culture inhibited the response by 85 to 90%. In contrast, anti-L3T4 did not inhibit the response in the presence of spleen AC. These results suggest that the molecules involved in T cell-AC interactions may vary depending on the AC source. Moreover, at least one of the putative target ligands for L3T4 presumably is not Ia, because anti-L3T4 inhibited T cell stimulation when Ia-negative AC were used.

Antigen recognition by H-2-restricted T cells. II. A tryptic ovalbumin peptide that substitutes for processed antigen

A 17-amino acid tryptic peptide of chicken ovalbumin, designated P323-339, that substituted for processed antigen when presented by glutaraldehyde prefixed accessory cells to specific I-restricted T hybridomas was characterized. The peptide antigen could not be demonstrated to have any specific or stable interactions with accessory cell Ia antigens by either direct binding or functional assays for inhibition of specific T cell activation. In addition, the T cell receptor for I-restricted antigen had no affinity for free antigen alone. A rabbit antibody specific for the antigenic peptide inhibited presentation when introduced before but not after binding of the peptide to accessory cells. These results extend our earlier finding that accessory cell-mediated processing of chicken ovalbumin can be completely explained by the fragmentation of the native molecule into smaller m.w. peptides, and suggests that if an antigen/Ia complex is important in T cell activation, it forms significantly only in the presence of the T cell receptor for I-restricted antigen.