Herpes-simplex-virus-specific,H-2Dk-restricted T lymphocytes bear receptors for H-2Dd alloantigen

Cytotoxic T lymphocytes generated in the course of an HSV-infection of CBA (H-2 ^k ) mice not only lyse syngeneic, virus-infected target cells but also cross-react with noninfected target cells expressing the D^d alloantigen. On the effector cell level, this alloreactivity is mediated by virus-specific CTL's that are restricted to H-2D^k determinants. On the prekiller cell level, the anti-HSV-reactive T cells exhibiting cross-reactivity for D^d alloantigen could be positively selected on H-2^d spleen-cell monolayers. After differentiation into cytolytic effector cells, target cells expressing D^d alloantigens and syngeneic HSV-infected target were lysed with equal efficiency. The results imply that the phenomenon of H-2-restricted versus nonrestricted T-cell reactivity is not due to distinct T-cell subsets, but rather is dependent on the antigeneic determinants recognized.     

Lectin-dependent cell-mediated cytotoxicity: induction of a unique effector cell population.

Spleen cells from C57BL/6 (H-2^b) mice were assayed for their ability to mediate lectin-dependent (Con A, PHA) cell-mediated cytotoxicity (LDCC), following immunization with erythrocytes, bovine serum albumin, Bacillus Calmette Guerin, and allogeneic (H-2^d) P815 cells. Sensitization with viable, but not formaldehyde-fixed, P815 cells resulted in lectin-dependent lysis of syngeneic EL-4 cells. All other sensitization procedures failed to produce LDCC. Spleen cells from mice challenged with high (10⁸) doses of P815 cells were capable of mediating both direct (anti-P815) cytotoxicity and LDCC, while challenge with low (10⁴) doses of P815 cells produced strong LDCC reactivity in the apparent absence of direct cytotoxicity (DCMC). Characterization of the effector cells indicated that LDCC reactivity was mediated by an activated, non-adherent T cell population. The effector cells appear to be unique in that LDCC could be induced in the absence of DCMC, LDCC activity appeared prior to DCMC, and DCMC could be removed by adsorption on P815 monolayers without depleting LDCC reactivity.     

Genetic control of immune responses of inbred mice: Responses against terpolymers poly(glu57lys38ala5) and poly(glu54lys36ala10)

The immune response patterns of inbred and congenic strains of mice against terpolymers poly(glu⁵⁷lys³⁸ala⁵) and poly(glu⁵⁴lys³⁶ala¹⁰) have been studied. Initial recognition of the polymers is ascribed to ‘GA’ receptors (Ir-GA gene product) on T cells of mice ofH-2 haplotypes,a,b,f,k ands, and ‘GL’ receptors (Ir-GL gene product) of mice ofH-2 ^p,H-2 ^q andH-2 ^j haplotypes, and to GA and/or GL receptors of mice ofH- 2^d andH- 2^r haplotypes. The specificity of the antibody is directed predominantly against GL. The inability to elicit antibody with GA specificity has been ascribed to the lack of significant concentrations of GA sequences in the polymers to interact with appropriate receptors on B cells. The weakest responders were mice of H-2^b haplotype. F¹ hybrids (responders×nonresponders) were all responders demonstrating the dominant character of responsiveness. Wide variations in antibody levels produced among strains of mice of theH-2 ^k andH-2 ^b haplotypes are ascribed to genes not linked toH-2.     

Cytotoxic T-cell responses to H-Y:Ir genes and associative antigens map inH-2

The secondary cytotoxic responses to the male-specific antigen (H-Y) in mice showH-2 restriction so that the cytotoxic female cell must share the K- and/or D-end antigen with the male target cells. The association with the K and/or D end varies with differentH-2 haplotypes,e.g., H-2 ^b cytotoxic cells require the H-2D^b antigen(s) on the target cells, while cytotoxic cells fromH-2 ^b/H-2 ^d F₁ mice sensitized toH-2 ^d male cells kill only male targets having H-2K^d antigen(s). This association of H-Y with appropriate K/D antigens seems to be needed also in the induction of the cytotoxic response. Of the independent haplotypes, onlyH-2 ^b strains are capable of making secondary anti-H-Y responses and this trait seems to be dominant,i.e., the F₁ strains with oneH-2 ^b parent are able to produce anti-H-Y cytotoxic cells against both theH-2 ^b parent and the nonresponder parent. The mating of the two nonresponder strains may produce F₁ mice which are responders, thus suggestingIr gene complementation. Mapping data indicates that at least one of these complementary genes is located in theI-C region fork/s complementation.     

Expression of subregion and haplotype preference for H-2Kk restricted cytotoxic T-cell responses in vivo and at the level of the precursor frequencies

Several strains of mice bearing the H-2K^k allele were found to generate in vivo strong CTL responses against TNP-haptenated syngeneic cells, while several other strains of mice were found to generate comparably weak or no responses. C3H × DBA/2)F1 mice (H-2^k × H-2^d) and A/J mice with the recombinant haplotype $\text{H-2}{}^{\mathrm{<mtext>k</mtext><mtext>d</mtext>}}$ generated CTL responses in vivo that were completely restricted toward the H-2^k haplotype or the K end of the $\text{H-2}{}^{\mathrm{<mtext>k</mtext><mtext>d</mtext>}}$ haplotype, respectively. The CTL activity of C3H × DBA/2)F1 and A/J mice against haptenated H-2^k targets was found to be more than 25-fold higher than the CTL activity on H-2^d targets. The CTL responses in vitro under macroculture conditions showed, on the other hand, only a 3- to 6-fold higher cytotoxic activity against the haptenated H-2^k targets as compared with haptenated allogeneic or H-2^d targets; and limiting dilution experiments in microcultures revealed that the CTL precursor frequencies were only 2- to 3-fold smaller for TNP-haptenated H-2^d or haptenated allogeneic targets than for haptenated H-2^k target cells. This indicated that sufficient numbers of H-2^d-restricted and allorestricted CTL precursors were actually present in these strains, but did not develop detectable cytotoxic activity in vivo. The exceptional property of the H-2^k haplotype is, therefore, only partly determined by a difference in the CTL precursor frequencies, and to the larger extent determined at the level of the activation of the CTL response.     

Studies on the mechanism of transplantation tolerance: I. Do suppressor cells play a role in the induction and maintenance of neonatal transplantation tolerance?

Neonatal transplantation tolerance was induced in CBA (H-2^k) mice to A (H-2^a) mice by injection of (CBA × A)F₁ spleen cells. Animals carrying an A-skin test allograft for more than 4 months without any visible sign of rejection were considered to be permanently tolerant. Permanently tolerant CBA mice were given normal syngeneic spleen cells to abrogate the state of tolerance. Abrogation of tolerance was greatly facilitated by antithymocyte serum (ATS) treatment of tolerant mice prior to the normal syngeneic cell transfer. Survival of A allografts on normal, adult, ATS-treated CBA mice was significantly prolonged (and in many cases “adult” tolerance was achieved) by transfer of spleen cells of syngeneic mice made permanently tolerant at neonatal age. The possible role of the F₁-cell “contamination” in the tolerance-inducing effect of the transferred “tolerant” spleen cells was excluded. The results indicate that ATS-sensitive suppressor cells play a definite role in the induction, maintenance, and transfer of neonatally induced transplantation tolerance.     

Role of the H-2I region in the generation of an antiviral cytotoxic T-cell response in vitro

The participation of H-2I gene products in generating virus-specific proliferative and/or cytotoxic T-lymphocyte (CTL) responses was investigated. Spleen cells from mice infected with vaccinia virus were restimulated secondarily in vitro with syngeneic virus-infected peritoneal exudate cells (PEC) and then restimulated in tertiary cultures with virus-infected PEC from syngeneic and partially histoincompatible strains of mice. Based on the finding that comparable proliferative responses resulted when stimulating the responding cells were histocompatible at the H-2K, I, or D region of the major histocompatibility complex (MHC), the additively enhanced, but not potentiated, proliferation caused by identity at two or three H-2 regions was analyzed. Enhancement of proliferation followed when the H-2K/D components plus virus and the H-2I components plus virus were either on the same, or alternatively on two, stimulating cells. This suggests that H-2K, D, and I plus virus trigger distinct T-cell subsets. A virus-specific CTL response was generated in vitro when spleen cells from virus-primed mice and even unprimed mice were stimulated with cells sharing only H-2K and/or H-2D of the MHC. Identity at the H-2I region did not stimulate a CTL response, nor did it influence the magnitude of the $\text{K}\text{D}$ restricted response. Nevertheless, the presence of anti-Ia antiserum in cultures of syngeneic stimulators and responders inhibited the cytotoxic response to a great extent. Therefore, H-2I region products seem to participate in the generation of virus-specific CTL in vitro.     

Distinct genetic pattern of mouse susceptibility to thyroiditis induced by a novel thyroglobulin peptide

Experimental autoimmune thyroiditis (EAT), induced by thuroglobulin (Tg) and adjuvant, is major histocompatibility complex-controlled and dependent on Tg-reactive T cells, but the immunopathogenic T-cell epitopes on Tg remain mostly undefined. We report here the thyroiditogenicity of a novel rat Tg peptide (TgP2; corresponding to human Tg amino acids 2695–2713), identified by algorithms as a site of putative T-cell epitope(s). TgP2 causes EAT in SJL (H-2 ^s) but not in C3H or B10.BR (H-2 ^k), BALB/c (H-2 ^d), and B10 (H-2 ^b) mice. This reveals a new genetic pattern of EAT susceptibility, since H-2 ^k mice are known to be high reponders (susceptible) after Tg challenge. Following in vivo priming with TgP2, T cells from only SJL mice proliferated significantly and consistently to TgP2 in vitro, whereas TgP2-specific IgG was observed in all strains tested. Adoptive transfer of TgP2-primed SJL lymph node cells to naive syngeneic recipients induced a pronounced mononuclear infiltration of the thyroid, which was more extensive than that observed after direct peptide challenge. TgP2 is non-immunodominant, since priming of SJL mice with rTg did not consistently elicit T-cell responses to TgP2 in vitro and a TgP2-specific T-cell hybridoma did not respond to antigen presenting cells pulsed with rTg. The data support the notion that Tg epitopes need not be either iodinated or immunodominant in order to cause severe thyroiditis and that the genetic pattern of the disease they induce can be distinct from that of Tg-mediated EAT. Correspondence to: G. Carayanniotis.     

Cell-mediated immune response to syngeneic UV induced tumors: I. The presence of tumor associated macrophages and their possible role in the in Vitro generation of cytotoxic lymphocytes

A primary in vitro sensitization system employing a chromium release assay was utilized to investigate reactivity of murine spleen cells toward syngeneic ultraviolet (uv) light induced fibrosarcomas. These tumors are immunologically rejected in vivo when implanted into normal syngeneic mice but grow progressivly when implanted into syngeneic mice that had previously been irradiated with subcarcinogenic levels of uv light. Following appropriate sensitization, spleen cells from both normal and uv irradiated mice are capable of developing cytotoxic lymphocytes in vitro against the uv induced tumors. It was subsequently discovered that in situ uv induced tumors all contained macrophages of host origin that became demonstrable only after enzymatic dissociation of the tumor tissue. These macrophages were immunologically active in vitro as their presence in the stimulator cell population was necessary to achieve an optimum anti-tumor cytotoxic response following in vitro sensitization. Anti-tumor reactivity generated by mixing spleen cells and tumor cells in the absence of tumor derived macrophages could be greatly enhanced by the addition of normal syngeneic peritoneal macrophages. When in vitro anti-tumor reactivity of spleen cells from normal and uv treated mice was compared under these conditions we again found no significant difference in the magnitude of the responses. In addition, the cytotoxic cells generated in response to uv induced tumors appeared to be highly cross reactive with respect to their killing potential. Cross reactive killing was observed between all uv induced tumors tested as well as with a syngeneic benz[a]pyrene (BP) induced fibrosarcoma. No cytotoxicity was observed against normal syngeneic PEC's even through these cells were shown to be susceptible to lysis by anti-H-2^k effector cells. It was concluded that: (a) A significant number of host-derived macrophages are present in uv tumor tissue. (b) These macrophages are important for the in vitro generation of tumor specific cytotoxicity. (c) Spleen cells from uv treated mice are capable of recognizing and responding against uv tumor associated antigens in vitro. Cytotoxic effector cells generated in response to uv induced tumors appear to have specificity for tumor associated antigens (TAA) present on all uv tumors tested as well as a syngeneic BP induced tumor. The relationship between in vivo and in vitro reactivity against uv tumors is discussed.     

Cellular analysis of the phenotypic correction of the genetically controlled low immune response to the polyproline determinant by macrophages

SJL mice are high responders to the polyproline region of poly(Tyr,Glu) -polyPro-polyLys, (T,G)-Pro-L and of poly (Phe,Glu) -polyPro-polyLys, (Phe,G)-Pro-L, whereas DBA/1 mice are the low responders to this moiety. The low responsiveness of DBA/1 mice to polyproline could be enhanced by immunization with (T,G)-Pro-L 4 days after stimulation of peritoneal cells by thioglycolate. The same effect was observed when DBA/1 mice were immunized with 10⁷ syngeneic peritoneal exudate cells (PEC) preincubated in vitro with the immunogen. Similar treatments of SJL mice did not enhance the high response to polyproline, nor did it enhance low responses to other synthetic polypeptides tested.The enhancing effect of PEC on immunocompetent cells was established by transferring graded numbers of spleen cells together with 10⁷ PEC into irradiated syngeneic DBA/1 recipients. The effective cell type in the PEC was found to be the macrophage as the same results were observed with the adherent-cell population. Furthermore, the effect was not abolished after in vitro irradiation of PEC with 5000 R or by anti-θ treatment. In vivo irradiation of the PEC donors 2 days before the cells were harvested also did not influence the phenotypic correction of the low responsiveness.Transfer experiments in which graded inocula of either marrow cells or thymocytes from DBA/1 donors were transferred into syngeneic recipients in the presence of an excess of the complementary cell type together with PEC indicated that the enhancing effect was reflected in the bone-marrow-cell population only.     

H-2 and Background influences on tissue grafts across the H-Y barrier

Male liver was grafted to kidney beds in syngeneic female mice. Relative influences ofH-2 haplotype, genetic background or interaction ofH-2 haplotype with genetic background on anti-H-Y response were evaluated using 27 inbred strains carrying eightH-2 haplotypes of independent origin and three naturally occurring recombinants. Females ofH-2 ^b haplotype acutely rejected the male graft as is reported for other tissue graft systems. AnH-2 haplotype influence was found for all haplotypes studied, with a greater variation of immunologic response revealed by histological analysis of liver grafts than is demonstrated by skin grafts. Strains carryingH-2 ^k ,H-2 ^j andH-2 ^p haplotypes expressed the greatest range of immunological variability with responses ranging from graft proliferation to graft rejection. Strains carrying theH-2 ^d haplotype had the most consistent responses with little reaction to the graft. The strong immune response by SJL/J (H-2 ^s ) female mice to the H-Y antigen is not typical of otherH-2 ^s strains, but is compatible with the reported hyperresponsiveness of this strain to alloantigens.     

Multigenic control of immune responses of inbred mice against the terpolymers poly(Glu57Lys38Ala5) and poly(Glu54Lys36Ala10) and linkage withH-2 haplotype

Results of immunizations of recombinant inbred and congenic strains of mice with the random polymers poly(glu⁵⁷ lys³⁸ala⁵) or GLA⁵ and poly(glu⁵⁴lys³⁶ala¹⁰) or GLA¹⁰ indicate that there is an association of the responsiveness with theH-2 haplotype. Although the C57BL/6J mice (H-2 ^b haplotype) are “non responders”, the C57BL/6By originally derived from mice of the same haplotype are responders. The immune response pattern of recombinant strains carrying haplotypes derived by crossing over within theH-2 complex indicate that the responsiveness is under control of anIr gene which maps to the left of theIB subregion. Studies with the backcross mice indicated multigenic control of the responsiveness, with one locus beingH-2 linked and another locus segregating independently ofH-2.     

Effect of mouse thymic medullary epithelial cell line on thymocyte development and functional maturationin vivo

Thymic medullary type epithelial cell line (MTEC1), which expressed H-2D^d and Ia^d, was derived from BALB/c mouse. MTEC1 cells were introduced by intrathymic injection into irradiated H-2^b mice reconstituted with H-2^bxd F1bone marrow cells. Two months later, the injected MTEC1 cells were found to be still present in the recipient thymus. Splenocytes from chimeric mice, inin vitro functional assays, were analyzed to investigate whether the MTEC1 cellsin vivo could induce the production of H-2^d restricted antigen-specific T cells. The H-2^d restricted VSV-antigen specific proliferating and IL-2 producing T cells as well as H-2^d restricted influenza virus specific cytotoxic T cells were found in chimeric mice injected with MTEC1 cells, and these cells were shown to be tolerant to H-2^d selfantigen. On the contrary, H-2^d restricted antigen-specific and H-2^d self-antigen tolerant T cells were not shown in control mice injected with saline. These results suggest that intrathymically injected MTEC1 cells could induce T lineage cell development and functional maturation in the intact thymus. A hypothesis of “second thymic selection” in thymic medulla has been postulated and its implication discussed.     

Genetic control of sensitivity to Moloney leukemia virus in mice V. Role of H-2-linked genes in the control of anti-FMR cytolytic T lymphocytes

Three H-2-linked genes, Rmv1, Rmv2, and Rmv3 control the resistance of mice against Moloney virus (MLV)-induced leukemias. It has been shown previously that they function as immune response (Ir) genes regulating the level of antivirus antibodies. In the present experiments, the cell-mediated anti-tumor response has been studied in a series of inbred strains selected for their resistance or sensitivity to the MLV-induced disease. We failed to detect any relationship between resistance and sensitivity and the ability to produce cytolytic T lymphocytes (CTL) directed against the virus-induced FMR cell surface antigen. Furthermore, the role of each Rmv gene has been studied separately using congenic pairs of mice differing at only one of these loci: we failed to evidence any influence of these genes in the cell-mediated antitumor reactions as measured by the ability to lyse syngeneic FMR(+) target cells. Nevertheless a gene mapping in the D region of the MHC but probably different from Rmv3 controls the response of a subset of anti-FMR CTL restricted by the H-2K^k antigens, with higher response in H-2D^d than in H-2D^b animals. This observation confirms the existence of H-2D region associated Ir genes regulating the CTL-mediated antitumor immune responses by choosing the subset of responder CTL, and suggests that a fourth H-2-linked gene plays some role in the genetic control of the anti-FMR immune response.     

Anti-H-2 antibodies induced by syngeneic immunization

Alloreactive cytotoxic antibodies were induced in BALB/c mice by syngeneic immunization with normal lymphoid cells. Sixteen out of 41 mice produced antibodies with distinct anti-H-2 specificity. Anti-K^k antibodies were present in all positive sera, but the individual sera produced different reactivity patterns when tested on a panel ofH-2 haplotypes. Absorption and immunoprecipitation experiments confirmed the H-2 specificity of the syngeneic sera. We hypothesize that virus-modified H-2^d structures have triggered alloreactive B-cell clones to produce anti-H-2 antibodies.     

In vitro T-lymphocyte proliferative response to mouse thyroglobulin in experimental autoimmune thyroiditis

The in vitro lymphocyte proliferative response to mouse thyroglobulin (MTg) was studied in good and poor responder mice in relationship to in vivo antibody formation and thyroid infiltration. CBA(H-2^k) and BALB/c(H-2^d) mice were immunized in the hind footpads with MTg incorporated into complete Freund's adjuvant (CFA). At weekly intervals up to 28 days, groups of mice were sacrificed. Their popliteal lymph nodes were cultured in vitro for proliferative response to MTg and their antibody levels and thyroid involvement determined. In good responder CBA mice, the proliferative responses to MTg were strongest on Days 8 to 14, where they were 9- to 14-fold over control levels, depending on the day of harvest. The response declined to 2- to 4-fold over background on Days 21 to 28, although high antibody levels were present throughout this period. The proliferative response was abrogated by anti-Thy-1 treatment, indicating its dependence on T cells. In poor responder BALB/c mice, no significant proliferative responses to MTg were observed at any time, although the animals displayed moderate levels of MTg antibody. The responses to PPD, in contrast, were similar in both strains, usually being 4- to 7-fold above background. Thyroid infiltration, like the proliferative response to MTg, was observed only in CBA mice. Thus lymphocyte proliferation at 8 to 14 days represents a reliable, early in vitro correlate of autoimmune thyroiditis induced with CFA as adjuvant.     

Specificities of killing by T lymphocytes generated against syngeneic SV40 transformants: studies employing recombinants within the H-2 complex.

T lymphocyte effectors to syngeneic SV40-transformed cells, generated by secondary in vitro sensitization of immune spleen cells, lyse SV40 transformed targets that are syngeneic at the H-2 locus. In this study we have employed recombinants within the H-2 region to examine in detail this H-2 specificity. H-2b effectors were found to lyse SV40-transformed targets from recombinants bearing either H-2Kb or H-2Db.H-2k effectors recognized only SV40-transformed H-2Kk, and not H-2Dk target cells. By using the same protocol for sensitization, no effector cells could be detected in H-2d mice. Effectors generated in H-2 recombinant mice showed that the response capacity resides with K and D. For example, HTG, which is H-2d except at the D locus (H-2Db), produced effector cells specific for SV40-transformed H-2Db targets. Thus, the secondary in vitro response to SV40 transformants was found to depend only on the K and D alleles and not to be modified by the I region to any measurable extent.     

Studies on the specificity of in vitro-induced lymphocytotoxicity to methylcholanthrene-induced fibrosarcoma cell lines

Cytotoxic effector lymphocytes were induced by in vitro immunization of lymph node and spleen cells from CS7B16(H2^b) and Balb/c(H2^d) mice to syngeneic or allogeneic methylcholanthrene-induced fibrosarcoma (MCAF) cell lines. The T cell-dependent cytotoxicity was specific to target cell lines to which the lymphocytes were immunized in vitro. Normal fibroblasts as stimulator cells did not induce lymphocytotoxicity to syngeneic MCAF cells or to normal syngeneic fibroblasts. The results indicate that the in vitro-immunized lymphocytes recognize individual specific tumor-associated antigens of the MCAF cells. In experiments in which the lymphocytes were immunized in vitro to allogeneic MCAF cells, cytotoxic reactions to alloantigens, but not to tumor-associated antigens, were detected. Incubation with phytohemagglutinin (PHA) during the sensitization period modified the specificity of the cell-mediated lysis of MCAF cells: Allogeneic as well as syngeneic target cells were destroyed by these effector cells. PHA induced a nonspecific cytotoxic effect which increased the specific lysis of target cells. The cytotoxicity of the in vitro-immunized lymphocytes was inhibited by incubation with membrane protein preparations from the syngeneic MCAF cell lines. In contrast to the specificity of the cytotoxic effect to the different syngeneic cell lines, the membrane extract of one individual syngeneic MCAF cell line was able to inhibit the lymphocytotoxicity to all other syngeneic cell lines. Membrane protein preparations from allogeneic MCAF cells or from normal syngeneic fibroblasts were not inhibitory. The in vitro-immunized cytotoxic lymphocytes did not impair the tumor growth in vivo as could be demonstrated by passive transfer of the lymphocytes in a Winn assay.     

B-Cell responses to male-specific antigen(s) in mice

It has been found that B-cell responses to male-specific antigen(s) can be clearly demonstrated by reversed plaque assays. Female mice injected with syngeneic male spleen cells showed significant increases (greater than 100 × in some strains) in the number of immunoglobulin-secreting cells in lymph nodes draining the injection site. There was a variation in B-cell responsiveness between strains and this correlated only partially with previously reported T-cell responsiveness to the H-Y antigen. C57BL (H-2 ^b ) mice were among the most responsive, while CBA (H-2 ^k ), (CBA × C57BL)F₁, and BALB/c (H-2 ^d ) were all much less responsive. These results apparently open up a new approach to the investigation of B-cell responses to male-specific antigen(s).     

A novel MHC class I-related molecule expressed on mouse thymic stroma cells and mature lymphocytes

A monoclonal antibody (mAb) TP-3 has been established by immunizing rats with the BALB/c mouse thymic epithelial cell line TEL-2. The TP-3 antigen is expressed on stroma cells of thymus, spleen, and lymph node in syngeneic BALB/c mice (H-2 ^d ). This antigen is also expressed at a low level on the cell surface of immature thymocytes, and at a high level on mature T and B cells. In allogeneic mice such as C57BL/6 (H-2 ^b ) or C3H (H-2 ^k ), no cells expressed the TP-3 antigen. Using H-2 congenic mice, reactivity with mAb TP-3 was found to map to a region of H-2D ^d L ^d or between D ^d and Qa, suggesting that TP-3 is a major histocompatibility complex (MHC) class I antigen. However, immunoprecipitation analysis indicated that this antigen is not identical to the classical mouse class I molecules in terms of molecular size, antigenicity, and tissue distribution.