Interleukin 15 induces endothelial hyaluronan expression in vitro and promotes activated T cell extravasation through a CD44-dependent pathway in vivo.

T cell recruitment to extralymphoid tissues is fundamental to the initiation and perpetuation of the inflammatory state during immune and autoimmune responses. Interleukin (IL)-15 is a proinflammatory cytokine whose described functions largely overlap with those of IL-2. The latter is attributable in large part to its binding of the heterotrimeric receptor that contains the beta and gamma chains of the IL-2R in combination with an unique IL-15R alpha chain. However, unlike IL-2, IL-15 and its receptor have a wide tissue and cell type distribution, including endothelial cells. Here, we examine the effect of IL-15 on hyaluronan expression by endothelial cells, and investigate its role in vivo in promoting the extravasation of antigen-activated T cells through a CD44-dependent pathway. The expression of hyaluronan on primary endothelial cells and microvascular endothelial cell lines is induced by IL-15, whereas IL-2 has no such activity. Moreover, intraperitoneal administration of IL-15 or TNF-alpha in the absence of other exogenous proinflammatory stimuli allows the extravasation of superantigen-stimulated T cells into this site in vivo in a CD44-dependent manner. T cell recruitment induced by IL-15 requires expression of an intact IL-2R beta chain, indicating that IL-15 operates in this context through the traditional IL-15R. The results suggest that IL-15 can regulate endothelial cell function and thereby enables a CD44-initiated adhesion pathway that facilitates entry of activated T lymphocytes into inflammatory sites. They further demonstrate a novel role for IL-15 (distinct from any of IL-2) in regulating microvascular endothelial cell adhesive function help to understand the role of IL-15R expression on endothelium, and further support a central position for this cytokine in orchestrating multiple sequential aspects of T cell effector function and therefore chronic inflammatory processes.

Frequent biclonality and Ig gene alterations among B cell lymphomas that show multiple histologic forms.

Configurations of Ig gene DNA were examined in multiple biopsy specimens from seven cases of human B cell lymphoma that showed histologic differences among the specimens within each case. Analysis by Southern blot hybridizations with DNA probes for each of the three Ig loci revealed that the configurations of DNA within these loci were identical among the specimens in two of the cases. This result indicated the monoclonality of these lymphomas, despite differences in histology between biopsy specimens. In contrast, no common nongermline configurations of Ig gene DNA were detected among multiple biopsies in each of three other cases. Therefore, different histologies correlated with separate clones of proliferating B cells in these cases. In the last two cases, the configurations of light chain gene DNA were the same among biopsies in each case, consistent with a monoclonal origin in both lymphomas. However, differences were detected in the configuration of the heavy chain gene DNA. Analysis with a series of DNA probes of the mu heavy chain region indicated that the differences in the DNA configurations of the heavy chain genes from the biopsies probably arose from postrearrangement deletions of either the switch or constant regions of the mu gene. These studies indicate that, contrary to the conventional belief, individual tumors that contain different histologic types of lymphoma within the same patient frequently arise from separate clones of neoplastic cells. Furthermore, the heavy chain genes of monoclonal tumors may show postrearrangement deletions, often resulting from instability of DNA sequences within or around the mu switch region.

CD44 and its ligand hyaluronate mediate rolling under physiologic flow: a novel lymphocyte-endothelial cell primary adhesion pathway.

The extravasation of leukocytes from the blood into tissues occurs as a multistep process: an initial transient interaction ("rolling"), generally thought to be mediated by the selectin family of adhesion molecules, followed by firm adhesion, usually mediated by integrins. Using a parallel plate flow chamber designed to approximate physiologic flow in postcapillary venules, we have characterized a rolling interaction between lymphoid cells and adherent primary and cultured endothelial cells that is not selectin mediated. Studies using blocking monoclonal antibodies indicate that this novel interaction is mediated by CD44. Abrogation of the rolling interaction could be specifically achieved using both soluble hyaluronate (HA) and treatment of the adherent cells with HA-reactive substances, indicating that HA is the ligand supporting this rolling interaction. Some B and T cell lines, as well as normal lymphocytes, either constitutively exhibit rolling or can be induced to do so by phorbol ester or in vivo antigen activation. These studies indicate that CD44 and its principal ligand hyaluronate represent another receptor/carbohydrate ligand pair mediating a novel activation-dependent pathway of lymphocyte/endothelial cell adhesion.

The route of antigen entry determines the requirement for L-selectin during immune responses.

L-selectin, an adhesion molecule constitutively expressed on leukocytes, is important for primary adhesion and extravasation of lymphocytes at specialized high endothelial venules within lymph nodes and other leukocytes at sites of inflammation. We have generated L-selectin-deficient mice by targeted disruption, and have confirmed a previously reported phenotype which includes strikingly impaired contact hypersensitivity (CHS) responses to reactive haptens (Tedder, T.F., D.A. Steeber, and P. Pizcueta. 1995. J. Exp. Med. 181:2259-2264; Xu, J.C., I.S. Grewal, G.P. Geba, and R.A. Flavell. 1996. 183:589-598.). Since the mechanism of this impairment has not been clarified, we sought to define the stage(s) at which the CHS response is affected in L-selectin-deficient mice. We show that epidermal Langerhans cells in L-selectin-deficient mice are normal in number, migrate to peripheral lymph nodes appropriately, and are functional in presenting allogeneic and haptenic antigens. Moreover, T cells, as well as neutrophil and monocyte effector populations, are fully capable of entry into the inflamed skin sites in the absence of L-selectin. Thus, antigen presentation and effector mechanisms are intact in L-selectin deficient mice. In contrast, virtually no antigen-specific T cells can be found within draining peripheral nodes after a contact challenge, suggesting that the defect resides primarily in the inability of antigen-specific T cells to home to and be activated in these nodes. Indeed, L-selectin-deficient mice mount completely normal CHS responses when alternate routes of immunization are used. These studies pinpoint the lesion in CHS to a discrete stage of the afferent limb of the response, clarify the role of L-selectin on effector populations, and illustrate the critical importance of the route of antigen entry to the successful execution of an immune response.

Circulating activated platelets reconstitute lymphocyte homing and immunity in L-selectin-deficient mice.

Peripheral lymph nodes (PLN) are critical for immunologic memory formation in response to antigens that penetrate the skin. Blood-borne lymphocytes first encounter such antigens after they home to PLN through a multi-step adhesion process that is normally initiated by L-selectin (CD62L) in high endothelial venules (HEV). Since naive T cells can not enter PLN normally in L-selectin-deficient mice, a delayed type hypersensitivity response to cutaneously applied antigen cannot be mounted. In this study, we report that the administration of activated platelets into the systemic circulation of L-selectin knockout mice restores lymphocyte trafficking to PLN, and reconstitutes T cell-mediated immunity in response to a cutaneous antigen. These effects required platelet-expressed P-selectin that allows activated platelets to transiently form a bridge between lymphocytes and HEV, thereby enabling lymphocytes to undergo subsequent beta2 integrin-dependent firm adhesion. These profound effects of platelet-mediated cell-cell interactions on lymphocyte trafficking and formation of immunologic memory may impact on a variety of autoimmune and inflammatory conditions.

Genomic organization of the selectin family of leukocyte adhesion molecules on human and mouse chromosome 1.

A structurally and functionally related group of genes, lymph node homing receptor (LHR), granule membrane protein 140 (GMP-140), and endothelial leukocyte adhesion molecule 1 (ELAM-1) are shown to constitute a gene cluster on mouse and human chromosome 1. In situ hybridization mapped GMP-140 to human chromosome 1 bands 21-24 consistent with chromosomal localization of LHR. Gene linkage analysis in the mouse indicated that these genes and serum coagulation factor V (FV) all map to a region of distal mouse chromosome 1 that is syntenic with human chromosome 1, with no crossovers identified between these four genes in 428 meiotic events. Moreover, long range restriction site mapping demonstrated that these genes map to within 300 kb in both the human and mouse genomes. These data suggest that LHR, ELAM-1, and GMP-140 comprise an adhesion protein family, the selectins, that arose by multiple gene duplication events before divergence of mouse and human. Furthermore, the location of these genes on mouse and human chromosome 1 is consistent with a close evolutionary relationship to the complement receptor-related genes, which also are positioned on the same chromosomes in both species and with which these genes share a region of sequence homology. These data characterize the organization of a genomic region that may be critical for intercellular communication within the immune system.

Requirement for CD44 in activated T cell extravasation into an inflammatory site.

Leukocytes extravasate from the blood into inflammatory sites through complementary ligand interactions between leukocytes and endothelial cells. Activation of T cells increases their binding to hyaluronate (HA) and enables CD44-mediated primary adhesion (rolling). This rolling could be induced in vivo in murine Vbeta8(+) T cells in response to specific superantigen stimulation; it was initially found in lymph nodes, then in peripheral blood, and finally within the peritoneum, the original inflamed site. The migration of Vbeta8(+) cells into the peritoneal cavity was dependent on CD44 and HA, as shown by inhibition studies. Thus, CD44-HA interactions can target lymphocytes to specific extralymphoid effector sites.

Mouse lymph node homing receptor cDNA clone encodes a glycoprotein revealing tandem interaction domains.

Isolation of a clone encoding the mouse lymph node homing receptor reveals a deduced protein with an unusual protein mosaic architecture, containing a separate carbohydrate-binding (lectin) domain, an epidermal growth factor-like (EGF) domain, and an extracellular precisely duplicated repeat unit, which preserves the motif seen in the homologous repeat structure of complement regulatory proteins and other proteins. The receptor molecule is potentially highly glycosylated, and contains an apparent transmembrane region. Analysis of messenger RNA transcripts reveals a predominantly lymphoid distribution in direct relation to the cell surface expression of the MEL-14 determinant, and the cDNA clone is shown to confer the MEL-14 epitope in heterologous cells. The many novel features, including ubiquitination, embodied in this single receptor molecule form the basis for numerous approaches to the study of cell-cell interactions.

The CD44-initiated pathway of T-cell extravasation uses VLA-4 but not LFA-1 for firm adhesion.

Leukocytes extravasate from the blood in response to physiologic or pathologic demands by means of complementary ligand interactions between leukocytes and endothelial cells. The multistep model of leukocyte extravasation involves an initial transient interaction ("rolling" adhesion), followed by secondary (firm) adhesion. We recently showed that binding of CD44 on activated T lymphocytes to endothelial hyaluronan (HA) mediates a primary adhesive interaction under shear stress, permitting extravasation at sites of inflammation. The mechanism for subsequent firm adhesion has not been elucidated. Here we demonstrate that the integrin VLA-4 is used in secondary adhesion after CD44-mediated primary adhesion of human and mouse T cells in vitro, and by mouse T cells in an in vivo model. We show that clonal cell lines and polyclonally activated normal T cells roll under physiologic shear forces on hyaluronate and require VCAM-1, but not ICAM-1, as ligand for subsequent firm adhesion. This firm adhesion is also VLA-4 dependent, as shown by antibody inhibition. Moreover, in vivo short-term homing experiments in a model dependent on CD44 and HA demonstrate that superantigen-activated T cells require VLA-4, but not LFA-1, for entry into an inflamed peritoneal site. Thus, extravasation of activated T cells initiated by CD44 binding to HA depends upon VLA-4-mediated firm adhesion, which may explain the frequent association of these adhesion receptors with diverse chronic inflammatory processes.

Functional activation of lymphocyte CD44 in peripheral blood is a marker of autoimmune disease activity.

Interactions between complementary receptors on leukocytes and endothelial cells play a central role in regulating extravasation from the blood and thereby affect both normal and pathologic inflammatory responses. CD44 on lymphocytes that has been "activated" to bind its principal ligand hyaluronate (HA) on endothelium can mediate the primary adhesion (rolling) of lymphocytes to vascular endothelial cells under conditions of physiologic shear stress, and this interaction is used for activated T cell extravasation into an inflamed site in vivo in mice (DeGrendele, H.C., P. Estess, L.J. Picker, and M.H. Siegelman. 1996. J. Exp. Med. 183:1119-1130. DeGrendele, H.D., P. Estess, and M.H. Siegelman. 1997. Science. 278:672-675. DeGrendele, H.C., P. Estess, and M.H. Siegelman. 1997. J. Immunol. 159: 2549-2553). Here, we have investigated the role of lymphocyte-borne-activated CD44 in the human and show that CD44-dependent primary adhesion is induced in human peripheral blood T cells through T cell receptor triggering. In addition, lymphocytes capable of CD44/HA-dependent rolling interactions can be found resident within inflamed tonsils. In analysis of peripheral bloods of patients from a pediatric rheumatology clinic, examining systemic lupus erythematosus, and a group of chronic arthropathies, expression of CD44-dependent primary adhesion strongly correlates with concurrent symptomatic disease, with 85% of samples from clinically active patients showing elevated levels of rolling activity (compared with only 4% of inactive patients). These rolling interactions are predominantly mediated by T cells. The results suggest that circulating T lymphocytes bearing activated CD44 are elevated under conditions of chronic inflammation and that these may represent a pathogenically important subpopulation of activated circulating cells that may provide a reliable marker for autoimmune or chronic inflammatory disease activity.

Eosinophil recruitment to the lung in a murine model of allergic inflammation. The role of T cells, chemokines, and adhesion receptors.

Eosinophil accumulation is a distinctive feature of lung allergic inflammation. Here, we have used a mouse model of OVA (ovalbumin)-induced pulmonary eosinophilia to study the cellular and molecular mechanisms for this selective recruitment of eosinophils to the airways. In this model there was an early accumulation of infiltrating monocytes/macrophages in the lung during the OVA treatment, whereas the increase in infiltrating T-lymphocytes paralleled the accumulation of eosinophils. The kinetics of accumulation of these three leukocyte subtypes correlated with the levels of mRNA expression of the chemokines monocyte chemotactic peptide-1/JE, eotaxin, and RANTES (regulated upon activation in normal T cells expressed and secreted), suggesting their involvement in the recruitment of these leukocytes. Furthermore, blockade of eotaxin with specific antibodies in vivo reduced the accumulation of eosinophils in the lung in response to OVA by half. Mature CD4+ T-lymphocytes were absolutely required for OVA-induced eosinophil accumulation since lung eosinophilia was prevented in CD4+-deficient mice. However, these cells were neither the main producers of the major eosinophilic chemokines eotaxin, RANTES, or MIP-1alpha, nor did they regulate the expression of these chemokines. Rather, the presence of CD4+ T cells was necessary for enhancement of VCAM-1 (vascular cell adhesion molecule-1) expression in the lung during allergic inflammation induced by the OVA treatment. In support of this, mice genetically deficient for VCAM-1 and intercellular adhesion molecule-1 failed to develop pulmonary eosinophilia. Selective eosinophilic recruitment during lung allergic inflammation results from a sequential accumulation of certain leukocyte types, particularly T cells, and relies on the presence of both eosinophilic chemoattractants and adhesion receptors.

Activation and interaction of CD44 and hyaluronan in immunological systems.

Adhesive interactions between receptors on vascular endothelial cells (EC) and circulating leukocytes are pivotal in regulating leukocyte extravasation. Although primary adhesion of lymphocytes to EC has been primarily attributed to the selectin family of receptors, CD44 can also mediate this function when activated to bind its ligand hyaluronan (HA). Triggering through the T cell receptor induces activated CD44 and CD44-dependent primary adhesion in both human and mouse lymphocytes, and the interaction can mediate the extravasation of activated T cells into an inflamed site. Lymphocytes capable of CD44/HA-dependent primary adhesion are found in peripheral blood of some rheumatologic patients, and their presence is associated with concurrent symptomatic or active disease. Thus, circulating T cells bearing activated CD44 may represent a pathogenically important subpopulation of activated cells that is elevated under conditions of chronic inflammation. Together, these data add to the selectin and immunoglobulin gene families a new receptor/ ligand pair and further our understanding of their potential physiological role; i.e., antigen-specific T cell activation together with local vascular inflammation permits the CD44/HA interaction and subsequent T cell extravasation.

Human homologue of mouse lymph node homing receptor: evolutionary conservation at tandem cell interaction domains.

A cDNA clone homologous to the mouse lymph node homing receptor core protein (mLHRc) was isolated from a cDNA library derived from stimulated human peripheral blood lymphocytes. Human RNA blot analysis shows a tissue and cell-line distribution of transcript expression generally parallel to that seen in the mouse, with expression confined to lymphoid tissues and some cell lines. Genomic DNA analysis suggests a low-copy gene under high-stringency conditions. The complete nucleotide sequence predicts a mature protein of 334 amino acids, identical in length to mLHRc. The protein shows striking conservation globally between human and mouse sequences. In particular, all three genre of protein interaction domains identified in the mouse--an animal lectin domain, an epidermal growth factor (EGF)-like domain, and two homologous repeat units preserving the motif of complement regulatory proteins (CRP)--are present in the human protein (hLHRc), and maintain the same tandem arrangement. The lectin and EGF-like regions are the most homologous, while the CRP domains are less conserved between species. The two CRP units in hLHRc are distinct from those in mLHRc in that they are homologous to one another rather than identical, suggesting strong pressure for maintenance of two repeats in this molecule. hLHRc is distinct from other kinds of lymphocyte adhesion molecules represented by VLA-4 (integrin) or CD44/gp90Hermes and, together with mLHRc and two other recently described molecules having a similar domain motif, defines a novel class of adhesion molecules exhibiting distinct evolutionary features. We propose that hLHRc likely represents the protein core of the human homologue of mLHRc functionally as well as structurally.

Hyaluronan anchoring and regulation on the surface of vascular endothelial cells is mediated through the functionally active form of CD44.

CD44 on lymphocytes binding to its carbohydrate ligand hyaluronan can mediate primary adhesion (rolling interactions) of lymphocytes on vascular endothelial cells. This adhesion pathway is utilized in the extravasation of activated T cells from the blood into sites of inflammation and therefore influences patterns of lymphocyte homing and inflammation. Hyaluronan is a glycosaminoglycan found in the extracellular matrix and is involved in a number of biological processes. We have shown that the expression of hyaluronan on the surface of endothelial cells is inducible by proinflammatory cytokines. However, the manner through which hyaluronan is anchored to the endothelial cell surface so that it can resist shear forces and the mechanism of the regulation of the level of hyaluronan on the cell surface has not been investigated. In order to characterize potential hyaluronan receptors on endothelial cells, we performed analyses of cell surface staining by flow cytometry on intact endothelial cells and ligand blotting assays using membrane fractions. Hyaluronan binding activity was detected as a major species corresponding to the size of CD44, and this was confirmed to be the same by Western blotting and immunoprecipitation. Moreover, alterations in the surface level of hyaluronan after tumor necrosis factor-alpha stimulation is regulated primarily by changes in the cell surface levels of the hyaluronan-binding form of CD44. In laminar flow assays, lymphoid cells specifically roll on hyaluronan anchored by purified CD44 coated on glass tubes, indicating that the avidity of the endothelial CD44/hyaluronan interaction is sufficient to support rolling adhesions under conditions mimicking physiologic shear forces. Together these studies show that CD44 serves to anchor hyaluronan on endothelial cell surfaces, that activation of CD44 is a major regulator of endothelial surface hyaluronan expression, and that the non-covalent interaction between CD44 and hyaluronan is sufficient to provide resistance to shear under physiologic conditions and thereby support the initial steps of lymphocyte extravasation.

Selective requirements for leukocyte adhesion molecules in models of acute and chronic cutaneous inflammation: participation of E- and P- but not L-selectin.

Adhesion molecules borne by both endothelial cells and circulating leukocytes are in large measure responsible for guiding the process of extravasation. The selectin family has been primarily associated with the early stages of adhesion involving initial contact and rolling. A significant body of evidence has accumulated indicating a fundamental role for the endothelial members of this family, E- and P-selectin, in a variety of inflammatory states and models. Although originally identified as the lymph node-specific lymphocyte homing receptor, L-selectin has also been suggested to play an important role in leukocyte recruitment to sites of inflammation. We have recently demonstrated, using L-selectin-deficient mice, that defects in contact hypersensitivity (CHS) responses are in essence due to the inability of T cells to home to and be sensitized within peripheral lymph nodes, whereas nonspecific effector cells are fully capable of entry into sites of cutaneous inflammation (Catalina et al, J Exp Med 184:2341, 1996). In the present study, we perform an analysis of adhesion molecule usage in two models of skin inflammation and show in both L-selectin-deficient as well as wild-type mice that a combination of P- and E-selectin is crucial for the development of both acute (croton oil) and chronic (contact hypersensitivity) inflammation at sites of the skin, whereas L-selectin does not appear to play a significant role. Moreover, alpha4 integrins are shown to be integral to a CHS but not an acute irritant response, whereas CD44 does not significantly contribute to either. These results provide a systematic examination in one study of major adhesion molecules that are critical in acute and chronic skin inflammation. They reinforce the essential role of the collaboration of E- and P-selectin in both specific and nonspecific skin inflammatory responses and the importance of alpha4 in the specific response only. In addition, they substantiate only a limited role, if any, for L-selectin in these cutaneous effector mechanisms and demonstrate the essential equivalence in this analysis of L-selectin-deficient mice compared with normal mice treated with blocking antibodies.

CD44 activation and associated primary adhesion is inducible via T cell receptor stimulation.

A defining juncture in the life of a T cell is its encounter with its cognate Ag, resulting finally in effector and/or memory T cells known to express, among other characteristics, increased surface levels of CD44. The requirements for the "activation" of CD44 to bind its major ligand, hyaluronan (HA), and the in vivo role of this interaction remain unresolved. We have recently proposed that the CD44/HA interaction is involved in primary lymphocyte adhesion, leading to extravasation at inflammatory sites. We show here that activation of CD44 and ability to engage in rolling occurs directly through polyclonal as well as Ag-specific TCR-initiated signaling. Using a superantigen, it is primarily the Ag-specific activated Vbeta-bearing cells that are induced to bind HA. In addition, this CD44 activation does not appear to be the result of overt changes in glycosylation. These results connect activation of CD44 on T cells with the initiation of immune responses and suggest potential roles for the CD44/HA interaction.

Plastocyanin: possible significance of quaternary structure.

Some properties of the blue copper protein plastocyanin from the green alga Scenedesmus have been investigated and compared with that from spinach, including amino acid composition, isoelectric point and copper content. The protein from Scenedesmus contains two, that from spinach four copper atoms per molecular weight of 40000. A combination of sodium dodecylsulfate/polyacrylamide gel electrophoresis and quantification of sulfhydryl groups indicates a strong preference for a species composed of 4 polypeptide chains of identical amino acid composition representing the enzymically active entity. Due to various treatments the subunits of both plastocyanins are detected as either monomer species alone or as monomer and dimer in a molar ratio of 2 : 1 on sodium dodecylsulfate gels. The four -SH groups per molecule are of different reactivity: two -SH groups can be detected after destruction of the chromophore; two more (forming an S-S bridge in the dimer) become evident after appropriate reduction. A KCN treatment for production of apoprotein is reported and the use of electrodialysis to improve incomplete apoprotein formation. These studies lend support to the proposal of a quaternary structure. Apoproteins were subjected to dodecylsulfate gel analysis, which proved to be an effective means of estimating both the extent of apoprotein formation and its reconstitution to the haloprotein.

The mouse lymph node homing receptor is identical with the lymphocyte cell surface marker Ly-22: role of the EGF domain in endothelial binding.

The lymph node homing receptor core polypeptide (mLHRc) is composed of a tandem collection of domains: a lectin domain, an epidermal growth factor (EGF) domain, and two repeats common in complement regulatory proteins. Here we demonstrate localization of mLHRc to chromosome 1, the portion syntenic with chromosome 1 in man. This locus is inseparable in mouse strains from the murine lymphocyte cell surface marker Ly-22. The data indicate that Ly-22 is an allelic determinant on the LHR resulting from a single amino acid interchange within the EGF domain. Cross-blocking experiments demonstrate that anti-Ly-22 and MEL-14 recognize independent epitopes and that Ly-22 is distinct from the carbohydrate binding region. Application of anti-Ly-22 in the in vitro binding assay shows inhibition of binding of lymphocytes to high endothelial venules (HEVs). The localization of the Ly-22 epitope in this novel chimeric protein suggests direct participation of the EGF domain in the adhesion of lymphocytes to HEV.

Structural studies on the murine Ia alloantigens. IV. NH2-terminal sequence analysis of allelic products of the I-A and I-E/C subregions.

Murine Ia alloantigens encoded by the I-A and I-E/C subregions were isolated from radiolabeled splenic lysates and examined by NH2-terminal sequence analysis. Haplotype-associated sequence variation was detected in the beta, but not alpha, subunits of both the A and E/C alloantigens. The A beta-polypeptides from k and b haplotypes show four differences in the 12 positions compared, whereas the E/C beta-polypeptides from k and r haplotypes show two differences in the 13 positions compared. No sequence variation was detected between the Ak and Ab alpha-chains (six positions compared) or between the E/Ck and E/Cr alpha-chains (11 positions compared). Homology relationships between these murine Ia alloantigens and the human Ia (DR) alloantigens are also presented.