Commitment of immature CD4+8+ thymocytes to the CD4 lineage requires CD3 signaling but does not require expression of clonotypic T cell receptor (TCR) chains.

As a consequence of positive selection in the thymus, immature CD4(+)8(+) double-positive, [DP] thymocytes selectively terminate synthesis of one coreceptor molecule and, as a result, differentiate into either CD4(+) or CD8(+) T cells. The decision by individual DP thymocytes to terminate synthesis of one or the other coreceptor molecule is referred to as lineage commitment. Previously, we reported that the intrathymic signals that induced commitment to the CD4 versus CD8 T cell lineages were markedly asymmetric. Notably, CD8 commitment appeared to require lineage-specific signals, whereas CD4 commitment appeared to occur in the absence of lineage-specific signals by default. Consequently, it was unclear whether CD4 commitment, as revealed by selective termination of CD8 coreceptor synthesis, occurred in all DP thymocytes, or whether CD4 commitment occurred only in T cell receptor (TCR)-CD3-signaled DP thymocytes. Here, we report that selective termination of CD8 coreceptor synthesis does not occur in DP thymocytes spontaneously. Rather, CD4 commitment in DP thymocytes requires signals transduced by either CD3 or zeta chains, which can signal CD4 commitment even in the absence of clonotypic TCR chains.

Lineage commitment in the thymus: only the most differentiated (TCRhibcl-2hi) subset of CD4+CD8+ thymocytes has selectively terminated CD4 or CD8 synthesis.

Lineage commitment is a developmental process by which individual CD4+CD8+ (double positive, DP) thymocytes make a decision to differentiate into either CD4+ or CD8+ T cells. However, the molecular event(s) that defines lineage commitment is controversial. We have previously proposed that lineage commitment in DP thymocytes can be molecularly defined as the selective termination of CD4 or CD8 coreceptor synthesis. The present study supports such a molecular definition by showing that termination of either CD4 or CD8 synthesis is a highly regulated event that is only evident within the most differentiated DP subset (CD5hiCD69hiTCRhibcl-2hi). In fact, essentially all cells within this DP subset actively synthesize only one coreceptor molecule. In addition, the present results identify three distinct sub-populations of DP thymocytes that define the developmental progression of the lineage commitment process and demonstrate that lineage commitment is coincident with upregulation of TCR and bcl-2. Thus, this study supports a molecular definition of lineage commitment and uniquely identifies TCRhibcl-2hi DP thymocytes as cells that are already committed to either the CD4 or CD8 T cell lineage.

Asymmetric signaling requirements for thymocyte commitment to the CD4+ versus CD8+ T cell lineages: a new perspective on thymic commitment and selection.

Differentiation of immature CD4+ CD8+ thymocytes into mature CD4+ CD8- and CD4-CD8+ T cells requires that synthesis of one or the other coreceptor molecule be terminated, a process referred to as lineage commitment. The present study has utilized a novel coreceptor reexpression assay to identify lineage commitment in immature thymocytes and has found that the MHC recognition requirements for CD4 commitment and CD8 commitment fundamentally differ from one another. Remarkably, we found that thymocyte commitment to the CD8+ lineage requires MHC class I-dependent instructional signals, whereas thymocyte commitment to the CD4+ lineage is MHC independent and may occur by default. In addition, an unanticipated relationship between lineage commitment and surface phenotype has been identified. These results are incompatible with current concepts and require a new perspective on lineage commitment and positive selection, which we refer to as asymmetric commitment.

Preferential expansion and activation of V beta 5+ CD4+ T cells in murine acquired immunodeficiency syndrome.

Infection of B6 mice with LP-BM5 MuLV results in a syndrome characterized by progressive and profound immunodeficiency, termed murine acquired immunodeficiency syndrome (MAIDS). In this report we show that preferential expansion and activation of V beta 5+ CD4+ spleen cells occurs early in the course of disease, which then progresses to more widespread polyclonal activation of CD4+ T cells. V beta 5 expansion occurs only on infection with MAIDS-associated MuLV and is not observed after infection with other MuLV. The possible role of V beta 5 activation and expansion as a necessary early step in the pathogenesis of MAIDS is discussed.

Adhesion of epidermal Langerhans cells to keratinocytes mediated by E-cadherin.

Langerhans cells (LC) are the principal accessory cells present in epidermis. Because LC have limited capacity for self-renewal, epidermis is continually repopulated by as-yet uncharacterized bone marrow-derived LC progenitors. In addition, although LC persist in epidermis for extended periods, LC are induced to migrate from skin to regional lymph nodes after antigen exposure. To begin to elucidate mechanisms involved in LC trafficking, we characterized LC-keratinocyte (KC) interactions. Here we report that fresh murine LC express cadherins, and that LC adhere to KC in vitro through E-cadherin. Cultured LC (which may bear a phenotypic and functional relationship to LC that have migrated to lymph nodes) express lower levels of E-cadherin and exhibit decreased affinity for KC. These results suggest that expression of E-cadherin by LC promotes persistence of these cells in epidermis, and that cadherins may play important and unanticipated roles in interactions between leukocytes and epithelia.

Characterization of the Mlsf system. I. A novel "polymorphism" of endogenous superantigens.

In addition to Mlsa (Mls-1a) and Mlsc (Mls-2a, Mls-3a), we and others have recently described a third set of stimulatory minor lymphocyte stimulating (Mls) determinants, which are ligands for "I-E related" V beta, V beta 5, V beta 11, and V beta 12. Although all V beta associated with the recognition of the conventional Mls determinants are, in general, uniformly deleted in those animals expressing relevant Mls, expression of Mlsf-related V beta reveals various deletion patterns among different strains. Here we describe extensive genetic studies to evaluate the relationship among the self-Ag responsible for clonal deletion of T cells bearing Mlsf-related V beta by using antibodies specific for TCR V beta chain. In addition, a panel of T cell clones specific for the Mlsf determinant were generated and employed to analyze the determinant specificity, which is recognized by Mlsf-reactive T cells in vitro as well as the role of class II molecules in T cell recognition of the Mlsf determinants. The results of these two independent approaches provide evidence that the Mlsf system is composed of a set of gene products that reveal a unique polymorphism in the induction of clonal deletion in vivo and in T cell activation in vitro. One of these gene products causes almost complete deletion of the self-Mlsf reactive T cell repertoire in vivo and elicits a strong proliferative response to Mlsf-specific T cell clones. Expression of the other gene products results in the clonal deletion of only part of the Mlsf-reactive T cell repertoire. Furthermore, the response pattern of Mlsf-specific clones to intra-MHC recombinant inbred strains and the inhibition pattern of these clones by anti-class II antibody suggested that although expression of the I-E molecule is essential for T cell recognition of Mlsf determinants, the A beta gene may also contribute to the efficient presentation of Mlsf determinants by forming unique class II E alpha A beta molecules.

Neuroendocrine responses to emotional stress: possible interactions between circulating factors and anterior pituitary hormone release.

We have shown that a psychological stressor can elicit increases in plasma AVP levels in normal human subjects. Since AVP can enhance the release of ACTH, and the pituitary gland is outside the blood-brain barrier, AVP present in the general circulation might extend the time course of stress-induced, CRF-mediated release of ACTH from the anterior lobe. Since PRA is involved in the synthesis of angiotensin I, the precursor of AII, and AII is known to enhance CRF-mediated release of ACTH from pituitary cells and to stimulate release of AVP, it is possible that the increase in PRA also contributed to the release of AVP and ACTH in this study. Reports differ as to whether circulating catecholamines can release ACTH in vivo by direct action on the pituitary. Finally, it has been reported that beta-EP enhances the release of PRL, and inhibits release of AVP. Since the increase in beta-EP in the present study was quite robust, it might have extended the PRL release, and truncated the AVP response.