Nature of the antigenic complex recognized by T lymphocytes. VII. Evidence for an association between TNP-conjugated macrophage membrane components and Ia antigens.

In the present study we examined the effects of anti-sera directed against guinea pig Ia antigens on the ability of TNP-conjugated macrophages to stimulate TNP-specific T lymphocyte proliferation. Treatment of macrophages with anti-Ia sera for 1 hr before, 1 hr immediately after, or as late as 24 hr after TNP-modification resulted in a reduced ability to stimulate the TNP-specific T cell. The inhibition produced by anti-Ia sera was specific and did not result from interference with the ability of macrophages to process TNP-conjugated membrane antigens in a nonspecific manner. Brief treatment with anti-Ia serum did not result in inhibition of Ia-antigen synthesis nor could evidence of carry-over of anti-Ia antibody into the lymphocyte cultures be obtained. These results demonstrate that anti-Ia sera interfere with the development of a TNP-specific immunogen on the macrophage surface and strongly suggest that an association exists between TNP-modified membrane proteins and Ia antigens on the macrophage surface.     

Macrophage-lymphocyte interaction. III. Site of alloantiserum inhibition of T lymphocyte proliferation induced by allogeneic or aldehyde-bearing cells.

Inhibition by anti-Ia sera of guinea pig T lymphocyte proliferation induced by allogeneic macrophages (MLR) and NaIO4 or neuraminidase-galactose oxidase-treated macrophages has been investigated in order to identify the target cell upon which the antisera act. Anti-2 and anti-13 alloantisera were found to inhibit both MLR and aldehydeinduced T cell reactivity when directed against the specificity of the stimulatory macrophage. Little or no inhibition was observed when these antisera were directed against the T lymphocyte specificity when cultures were harvested at the time of peak proliferation. In addition, anti-2 serum was found to inhibit macrophage-lymphocyte rosett formation at 20 hr between neuraminidase-galactose oxidase-treated strain 2 macrophages and strain 13 lymphocytes. These findings demonstrate that inhibition of T cell proliferation can be produced by anti-Ia sera directed against the macrophage and raise the possibility that Ir gene products may function in part at the level of the macrophage.     

Nature of the antigenic complex recognized by T lymphocytes. IV. Inhibition of antigen-specific T cell proliferation by antibodies to stimulator macrophage Ia antigens.

We have previously demonstrated that nonimmune guinea pig T lymphocytes could be specifically sensitized with TNP-modified allogeneic macrophages after eliminating the alloreactive T cells with bromodeoxyuridine (BUdR) and light treatment. This procedure allowed the unique opportunity to use anti-Ia sera directed against the Ia antigens of only the stimulator macrophages or responder T cells to determine against which cell type anti-Ia would block TNP-specific stimulation. It was found that the TNP-specific DNA synthetic response of BUdR and light-treated T cells stimulated with TNP-modified allogeneic macrophages was totally eliminated by anti-Ia sera directed solely against the allogeneic stimulator macrophage. In contrast, anti-Ia sera directed only against the responder T cells had no effect on their response to TNP-modified allogeneic macrophages. These findings indicate that macrophage Ia antigens are required for efficient T cell-macrophage interactions and raise the possibility that T cell Ia antigens may not be required for collaboration with macrophages. This latter possibility was substantiated by experiments in which we show that treating T cells with anti-Ia sera and complement to remove the Ia-positive cells either before or after priming, or both, had no effect on their ability to be primed and restimulated with TNP-modified macrophages.     

Antigen-specific murine T-cell proliferation: role of macrophage surface Ia and factors.

The proliferation of Mycobacterium-primed murine lymph node T cells to purified protein derivative of tuberculin (PPD), as measured by the uptake of tritiated thymidine, requires the obligatory participiation of macrophages which stimulate the T cells either directly with antigen in association with cell surface Ia (I region-defined antigens), or indirectly by means of soluble factors. We have examined the possibility that this functional dichotomy is due to heterogeneity within the macrophage population. Since the maturation of macrophages from the precursor monocytes is associated with cell enlargement, macrophage subpopulations differing in developmental stage are obtained by cell fractionation according to size by velocity sedimentation. Nylon-wool-purified T cells which have been depleted of macrophages and B cells are stimulated with PPD either in a free form or bound to macrophages which have been incubated for a short time (i.e., pulsed) with PPD. We found that for PPD-pulsed macrophages, only the smallest (and probably the most immature) are capable of inducing T-cell proliferation. This antigen presentation function is mediated by cell surface Ia since it is abolished by pretreatment of the macrophages with anti-Ia serum and complement. On the other hand, all macrophages, irrespective of sensitivity to anti-Ia serum, secrete factors which will stimulate T-cell proliferation in the presence of free PPD. Thus the maturation of macrophages is accompanied by a shift from Ia-dependent to Ia-independent mechanisms of immunostimulation.     

Specific absorption of T lymphocytes committed to soluble protein antigens by incubation on antigen-pulsed macrophage monolayers.

Monolayers of macrophages (Mphi) pulsed with antigen were used as immunosorbents for T lymphocytes from guinea pigs primed to soluble protein antigens. T lymphocytes were cultured on the Mphi monolayers for 4 hr, then aspirated and reincubated on a fresh monolayer pulsed with the same antigen for a second and a third step. T lymphocytes so treated were selectively deprived of cells responding in assay for antigen-dependent proliferation against the antigen used for pulsing the absorbing monolayer, but maintained their response to other antigens. The lymphocytes adhering to the Mphi of the absorbing monolayer were capable of giving a full response to the antigen used for pulsing the Mphi of the monolyers. The proliferative response of F1 T lymphocytes to antigen in association with Mphi of either parental strain could be absorbed leaving the response to antigen in association with Mphi of the other parental strain. The absorption of the proliferative response was not inhibited by addition of excess soluble antigen to the medium of the absorption culture. Our results indicate that specific guinea pig T lymphocytes responding by proliferation to soluble protein antigens recognize and bind specifically to a complex of Ia antigen and protein antigen at the surface of the Mphi.     

The expression of Ia antigens on immunocompetent cells in the quinea pig. I the differential expression of Ia antigens on T cell subpopulations.

We have examined the effect of negative selection with anti-Ia serum and C on a number of T cell functions and have clearly defined two subpopulations of guinea pig T lymphocytes. One subpopulation is susceptible to the lytic effects on anti-Ia serum and C and includes the majority of the primed T cells which proliferate and which produce migration inhibition factor in response to specific antigen stimulation in vitro. The lytic effects of anti-Ia serum were directed against the antigen-specific T cell and not an accessory cell such as a macrophage or nonantigen-specific T cell. No evidence for allelic exclusion of the Ia antigens of the antigen-responsive cell could be demonstrated. The susceptibility of the mitogen-responsive T cell to lysis by anti-Ia serum and C varied with the mitogen used, anatomic origin of the T cell, and the strain of animals studied. A second subpopulation of T cells is completely resistant to the lytic effects of anti-Ia serum and C and includes the primed T helper cell and the T cell that proliferates in response to alloantigenic stimulation in the MLR.     

The expression of Ia antigens on immunocompetent cells in the guinea pig. III. Functional selection of Ia-negative T cells by in vitro culture.

In the present study we examined the expression of I-region-associated (Ia) antigens by guinea pig T lymphocytes stimulated in vitro with antigen-pulsed macrophages. Treatment of lymph node (LNL) or peritoneal exudate (PEL) T cells taken directly from immune animals with anti-Ia serum and complement (C) dramatically reduced their proliferative response to antigen-pulsed macrophages when determined on the 4th day of culture. In contrast, the response of immune T cells that had been selected by culture for a week with antigen-pulsed macrophages and restimulated in a second culture was not affected by anti-Ia and C treatment. This same result occurred with selected LNL or PEL that were initially treated before the selection culture with either normal serum or anti-Ia serum and C. LNL became resistant to anti-Ia serum and C treatment by 3 days of culture whereas antigen-specific PEL were still sensitive at that time. These results indicate that in an immune animal two antigen-specific T cell subpopulations are generated based on their sensitivity to anti-Ia serum and C treatment, but that only the resistant population is selected by in vitro culture. In addition, we demonstrated that the Ig-negative T cell population can only be activated by histocompatible antigen-pulsed macrophages.     

Stimulation of T-cell activation by UV-treated, antigen-pulsed macrophages: evidence for a requirement for antigen processing and interleukin 1 secretion

The nature of the defect(s) in the ability of UV-treated guinea pig macrophages to stimulate the proliferative response of guinea pig T cells to soluble protein antigens was investigated. T cells proliferated vigorously when cultured with peritoneal exudate cells (PEC) which had been pulsed with soluble protein antigens, but failed to proliferate when cultured with soluble antigen or with antigen-pulsed, UV-treated PEC. UV-treated macrophages were unable to secrete interleukin 1 (IL-1). Addition of IL-1 partially restored the T-cell proliferative response stimulated by antigen-pulsed, UV-treated PEC. However, IL-1 was able to restore such a response only when the PEC were pulsed with antigen before being exposed to UV. Similar results were obtained when antigen-pulsed PEC were used to stimulate T cells to secrete interleukin 2 (IL-2). These results demonstrate that UV-treated macrophages are defective both in their ability to properly process and present antigen for T-cell recognition and in their ability to secrete IL-1.     

Genetically restricted immune responses in guinea pigs primed in vivo with antigen-bearing macrophages.

Guinea pigs injected intradermally with antigen pulsed macrophages generate a population of immune T cells that proliferate in vitro on second exposure to antigen. T cells from F1 (2 X 13) guinea pigs immunized with DNP-OVA on one parental macrophage respond in vitro only to DNP-OVA on macrophages identical to those used for immunization and not to DNP-OVA associated with the other parental macrophages. These results demonstrate that the immunogenicity of antigen is dependent upon the macrophages used for priming in that, with this approach, strain 2 or 13 guinea pigs immunized with allogeneic macrophages pulsed with antigen do not respond to either allogeneic or syngeneic antigen-bearing macrophages. However, lysates of antigen-pulsed macrophages can still immunize either allogeneic or syngeneic recipient via their own macrophages. F1 (2 X 13) guinea pigs are immunized by insulin B chain pulsed strain 13 macrophages (responder) but not by strain 2 macrophages (nonresponder) suggesting that whether a F1 (nonresponder X responder) guinea pig recognizes antigen bound to a parental macrophage is genetically restricted before immunization to the same extent as the donor parental macrophages used for immunization.     

The expression of Ia antigens on immunocompetent cells in the guinea pig. II. Ia antigens on macrophages.

Only 15 to 25% of purified oil-induced guinea pig macrophages could be lysed by treatment with anti-Ia serum and C. Those cells remaining alive after treatment were not damaged and were metabolically active since they readily phagocytized latex beads. However, the "Ia-negative" macrophages were markedly deficient in their ability to present protein antigens to immune T lymphocytes and to function as stimulator cells when mixed with allogeneic T cells in the mixed leukocyte reaction. It thus appears that Ia antigens are expressed on a subpopulation of macrophages and that this subpopulation plays a critical role in the activation of T cell proliferation to soluble protein antigens and to alloantigens.     

Functional and biochemical parameters of peptide antigen presentation

To understand the mechanism by which peptide antigens are processed and presented to T cells, we examined the T-cell response to the 13-amino-acid peptide alpha-melanocyte-stimulating hormone (alpha-MSH). To determine the fine specificity of T-cell recognition, T cells specific for alpha-MSH, and genetically restricted by I-Ab/d, were challenged with different alpha-MSH analogs and homologs. It was found that intact alpha-MSH, including the blocked amino and carboxy termini of the native molecule, was required for T-cell responsiveness. Antigen-presenting cells (APC) could be briefly pulsed with alpha-MSH and then present the alpha-MSH antigenic determinant to T cells, indicating that the relevant antigen was retained by the APC. APC stimulatory capacity was dramatically reduced by aldehyde treatment of the APC, or by pulsing the APC with alpha-MSH at low temperature. Efficient alpha-MSH pulsing was also impaired by treatment of the APC with the carboxylic ionophore, monensin, but not by the lysosomotropic agents chloroquine and methylamine. In addition, isolated APC plasma membranes added to the T cells in the presence of soluble alpha-MSH were not stimulatory. However, plasma membranes isolated from APC that had been previously pulsed with alpha-MSH retained stimulatory activity for T-cell responses. The only detectable alpha-MSH contained in these pulsed APC membranes was in an acid-stable complex of higher molecular weight than native peptide. The amount of alpha-MSH detected in the cellular membrane fraction isolated by density gradient sedimentation was also reduced by treatments that reduced the APC stimulatory capacity, such as pulsing at low temperature or in the presence of monensin. Taken together, these results suggest that processing of alpha-MSH is unlike that heretofore described for other peptide antigens and seems to involve APC handling to form the stimulatory moiety presented on the APC surface.     

Specific binding of T lymphocytes to macrophages. V. The role of Ia antigens on Mphi in the binding.

The effect of anti-Ia alloantiserum on the capacity of selected peritoneal exudate lymphocytes (selected PEL) to bind to antigen-pulsed F1 (responder x nonresponder) macrophages was investigated. With the use of selected PEL for antigens under Ir gene control, it was shown that anti-Ia serum to the responder haplotype blocked adherence of selected PEL to antigen-pulsed macrophages whereas anti-Ia serum to the nonresponder haplotype did not. The target cell of the anti-Ia alloantiserum appeared to be the macrophage because anti-13 Ia in contrast to anti-2 Ia did not inhibit binding of F1 (2 x 13) DNP-GL selected PEL to DNP-GL pulsed strain-2 Mphi (responder strain). Taken together with previous experiments that indicate that an antibody to the native protein antigen employed is unable to block specific binding, the present results suggest that T cells may recognize fragments of exogenous antigen in association with Ia molecules.     

T lymphocyte-enriched murine peritoneal exudate cells. III. Inhibition of antigen-induced T lymphocyte Proliferation with anti-Ia antisera.

The recent development of a reliable murine T lymphocyte proliferation assay has facilitated the study of T lymphocyte function in vitro. In this paper, the effect of anti-histocompatibility antisera on the proliferative response was investigated. The continuous presence of anti-Ia antisera in the cultures was found to inhibit the responses to the antigens poly (Glu58 Lys38 Tyr4) [GLT], poly (Tyr, Glu) ploy D,L Ala-poly Lys [(T,G)-A--L], poly (Phe, Glu)-poly D,L Ala-poly Lys [(phi, G)-A--L], lactate dehydrogenase H4, staphylococcal nuclease, and the IgA myeloma protein, TEPC 15. The T lymphocyte proliferative responses to all of these antigens have previously been shown to be under the genetic control of major histocompatibility-linked immune response genes. The anti-Ia antisera were also capable of inhibiting proliferative responses to antigens such as PPD, to which all strains respond. In contrast, antisera directed solely against H-2K or H-2D antigens did not give significant inhibition. Anti-Ia antisera capable of reacting with antigens coded for by genetically defined subregions of the I locus were capable of completely inhibiting the proliferative response. In the two cases studied, GLT and (T,G)-A--L, an Ir gene controlling the T lymphocyte proliferative response to the antigen had been previously mapped to the same subregion as that which coded for the Ia antigens recognized by the blocking antisera. Finally, in F1 hybrids between responder and nonresponder strains, the anti-Ia antisera showed haplotype-specific inhibition. That is, anti-Ia antisera directed against the responder haplotype could completely block the antigen response controlled by Ir genes of that haplotype; anti-Ia antisera directed against Ia antigens of the nonresponder haplotype gave only partial or no inhibition. Since this selective inhibition was reciprocal depending on which antigen was used, it suggested that the mechanism of anti-Ia antisera inhibition was not cell killing or a nonspecific turning off of the cell but rather a blockade of antigen stimulation at the cell surface. Furthermore, the selective inhibition demonstrates a phenotypic linkage between Ir gene products and Ia antigens at the cell surface. These results, coupled with the known genetic linkage of Ir genes and the genes coding for Ia antigens, suggest that Ia antigens are determinants on Ir gene products.     

The activation of guinea pig T lymphocytes by anti-beta 2-microglobulin serum.

In order to study further the role of beta 2-m in the regulation of the immune response, we have examined the effects of a goat anti-guinea pig beta 2-m serum on a number of T lymphocyte functions in vitro. Anti-beta 2-m serum produced a marked inhibition of the response of peritoneal exudate T cells to antigen and mitogen stimulation. Surprisingly, a marked activation of lymph node T lymphocyte proliferation was observed in the absence of antigen or mitogen stimulation. This stimulatory effect of anti-beta 2-m serum was shown to be specific for beta 2-m and required the presence of macrophages. The T cell proliferative response induced by anti-beta 2-m could not be blocked by antisera to the antigens of the guinea pig MHC. These studies suggest that beta2-m may play some critical role in the immune response at the level of T cell activation.     

Adherent cell function in murine T lymphocyte antigen recognition. I. A. macrophage-dependent T cell proliferation assay in the mouse.

The data in this report describe a T cell proliferation assay with nylon wool column-purified murine lymph node lymphocyte from animals immunized by footpad injection of antigen in CFA. It was found that the in vitro immune response of sensitized T cells to soluble protein antigens was functionally dependent on the presence of adherent cells, more specifically macrophages, at all concentrations of in vitro antigen challenge. The response was due to T cells in that cytotoxic treatment of the immune lymphocyte cells with anti-Thy 1.2 serum and complement effectively eliminated the antigen-specific DNA synthetic responses. The antigen-specific proliferation of murine lymphocytes depleted of adhereent cells could not be reconstituted with either guinea pig macrophages nor murine fibroblasts, indicating the existence of species and cell type specificity. In contrast to previous observations in the guinea pig, soluble products of cultured adherent cells could at least partially replace the function of intact macrophages in the response to antigen.     

Anti-Ia inhibits the activity of B cells but not a T cell-derived helper mediator

The effects of addition of anti-Ia sera to cultures of B cells responding to a number of different stimuli were studied. Anti-Ia sera inhibited the responses of B cells to thymus-dependent and thymus-independent antigens. The effects of antisera directed at different subregions within theI region were examined, using the same anti-Ia serum and mouse strains congenic atI. There was some indication that antisera directed at theI-C region might be more efficient at inhibiting responses to a thymus-independent antigen than to a thymus-dependent antigen. An antiserum to another B-cell surface component controlled by theH-2 complex, the D glycoprotein, had no effect on the response of B cells to a thymus-dependent antigen. By contrast, anti-Ia serum added to cultures had no effect on the activity of a T cell-derived, nonspecific, B-cell helper mediator (NSM). We concluded that the binding of anti-Ia sera to B-cell surfaces inhibited B-cell responses to antigen, either by competing directly with the binding of signal molecules, or by delivering an inhibitory signal to the B cell, such that it was subsequently refractory to stimulatory signals.     

Antigen handling by guinea pig macrophages: further evidence for the sequestration of antigen relevant for activation of primed T lymphocytes.

Guinea pig macrophages can take up sufficient 2,4 dinitrophenyl guinea pig albumin during a brief in vitro exposure at 37 degrees C to trigger proliferation and lymphokine production with primed T lymphocytes on subsequent co-culture. Treatment of such antigen-bearing macrophages with trypsin, a procedure which removes surface antigen, does not alter the ability of such macrophage to initiate the release of migration inhibition factor from sensitized T lymphocytes. In addition, formation of antigen-specific rosettes between primed T cells and antigen-bearing macrophages is not blocked by high concentrations of antibody directed against the antigen mediating this interaction. Similarly, primed T lymphocyte DNA synthesis induced by antigen-bearing macrophages is not inhibited by specific antibody to that antigen. These data support the conclusion that the fraction of macrophage-associated antigen which is relevant to T lymphocyte activation does not reside on the macrophage surface but rather remains in a restricted compartment from which it is accessible to the T cell but unavailable to either blockade by specific antibody or removal by proteolytic enzymes.     

Functional analysis of cloned macrophage hybridomas. IV. Induction and inhibition of mixed lymphocyte responses

A series of macrophage (M phi) hybridomas were generated by fusion of drug-marked P388D1 (H-2d) tumor cells with CKB (H-2k) splenic adherent cells. The ability of this panel of cloned M phi hybridomas expressing various levels of surface Ia antigens to induce allogeneic mixed lymphocytes responses (MLR) was examined. All MLR stimulatory M phi hybridomas expressed surface Ia antigens. However, some Ia+ and all Ia- M phi hybridomas were unable to induce vigorous MLR responses. Furthermore, even after induction of surface Ia antigen expression with Con A supernatants (Con A Sn) or purified interferon-gamma, the nonstimulatory M phi hybridomas remained ineffective at inducing strong MLR proliferative responses. Furthermore, addition of the latter M phi hybridoma clones (both with and without Con A Sn treatment) to conventional MLR cultures resulted in inhibition of MLR responses. The series of inhibitory M phi hybridomas secreted normal levels of IL 1 upon stimulation with lipopolysaccharide. After surface Ia induction with Con A Sn, the inhibitory M phi hybridomas could stimulate secretion of IL 2 and expression of IL 2 receptors. Moreover, although they inhibited conventional MLR responses, IL 2 production and IL 2 receptor expression were not significantly inhibited. Addition of these M phi hybridomas 24 to 48 hr after initiation of MLR response also inhibited MLR proliferation. The results indicated that the group of inhibitory M phi hybridomas can inhibit MLR responses after IL 2 secretion and acquisition of IL 2 receptors. Finally, this inhibitory activity has been maintained during 1 yr of continuous in vitro culture, and the hybridomas represent a stable "homogeneous" subpopulation of inhibitory macrophages. Thus, the inhibitory phenotype appears to reflect arrest at a distinct differentiation stage.     

Naturally soluble tumor antigens from guinea pig hepatomas: isolation and partial characterization.

Naturally soluble tumor antigens were detected in the ascites fluid of guinea pigs bearing an ascites tumor and from exhausted tissue culture media of cultured tumor cells. Two antigenically distinct cell lines of diethylnitrosamine-induced strain-2 guinea pig hepatomas (line-10 and line-1) served as the source of tumor antigens. Tumor antigen activity was detected by four different techniques: immunodiffusion, inhibition of complement-mediated cytotoxicity, inhibition of membrane immunofluorescence, and delayed cutaneous hypersensitivity. With syngeneic tumor-specific antiserum, line-10 guinea pig tumor antigens were detected by immunofluorescence in the concentrated ascites and tissue culture fluids. With a xenogenic antiserum, demonstrated to be tumor specific, line-10 tumor antigens were detected not only in the concentrated ascites and tissue culture fluids but also in two of the partially purified fractions of these fluids. When the line-10 concentrated ascites and its fraction I were subjected to ultracentrifugation at 300,000 x G for 1 hr, the antigen activity was retained in the supernatant and thus by this criterion the tumor antigens detected in these samples are soluble. Immunodiffusion data indicate that more than one antigen is present in the line-10 system since three lines of precipitation were detected when line-10 concentrated ascites was reacted with the line-10 tumor-specific antiserum. In contrast to this, the line-10-concentrated tissue culture fluid displayed only one line of precipitation. Although tumor antigens could not be demonstrated in the other antigenically distinct tumor cell line, line-1, by immunodiffusion or inhibition of membrane immunofluorescence, inhibition of complement-mediated cytotoxicity was able to detect tumor antigens in the line-1 concentrated ascites and tissue culture fluids.     

Analysis of murine major histocompatibility complex class II-restricted T-cell responses to the flavivirus Kunjin by using vaccinia virus expression.

The present paper analyzes the influence of major histocompatibility complex (MHC) class II (Ir) genes on MHC class II-restricted T-cell responses to West Nile virus (WNV) and recombinant vaccinia virus-derived Kunjin virus antigens and identifies the immunodominant Kunjin virus antigens. Generally, mice were primed by intravenous infection with WNV or Kunjin virus, and their CD4+ T cells were stimulated in vitro 14 days later with WNV or Kunjin virus antigens to pulse macrophage or B-cell antigen-presenting cells (APC). WNV-specific in vitro T-cell responses from H-2b mice were higher than those from H-2d, H-2k, and H-2q mice. When recombinant vaccinia virus-derived Kunjin virus antigen preparations were tested in vitro, Kunjin virus-immune T cells of H-2b haplotype responded most strongly to structural (prM, C, E) and membrane-associated nonstructural (NS1) proteins encoded by VKV 1031 and showed weaker responses to cytosolic nonstructural protein NS5 (VKV 1022), whereas the responders of H-2k haplotype responded most strongly to the antigens encoded by VKV 1022 and gave lesser responses to VKV 1031. H-2d T cells gave weaker responses than either H-2b or H-2k cells, with responses to VKV 1031 generally being higher than those to VKV 1022. Responses to VKV 1023 or VKV 1024 encoding all of the NS3 to NS5 gene sequence or to VKV 1023 encoding all of NS3 were weak or absent. Within a given inbred strain, B cells and macrophages differed in their abilities to present recombinant vaccinia virus-derived Kunjin virus antigens, both in terms of magnitude of T-cell responses induced and the particular Kunjin virus protein presented. T cells from different non-MHC genetic backgrounds varied in their requirements of macrophage numbers as APC for maximum reactivity, suggesting that the concentration of class II MHC antigens and other molecules affecting APC-T-cell interaction varied in mice with different genetic backgrounds. Regardless of MHC haplotype, responses to VKV 1024, which encompasses VKV 1023 and VKV 1022, were either absent or lower than those to VKV 1022, possibly reflecting differences in the processing requirements of these two proteins. When mice were primed intravenously with recombinant vaccinia virus and when their CD4+ T cells were stimulated in vitro with native Kunjin virus antigens, VKV 1031 primed more efficiently than Kunjin virus and VKV 1022 primed similarly to Kunjin virus.