A fail-safe mechanism for maintaining self-tolerance

Lymphocyte proliferation in mice congenitally deficient in T-cell receptor alpha beta + cells

In mice and humans, T cells are characterized on the basis of T-cell receptor (TcR) expression and divided into the major TcR alpha beta + and minor TcR gamma delta + populations. TcR alpha beta + cells are considered to be the primary regulators of the immune response, whereas the function of TcR gamma delta + cells is unclear. Mice congenitally deficient in TcR alpha beta-expressing cells provide an ideal model for analyzing the independent in vivo function of TcR gamma delta + cells in the absence of TcR alpha beta + cells. Here we report that lymphoid organs in TcR alpha mutant mice undergo substantial enlargement after being challenged by environmental antigens. This organ expansion can be attributed in part to increases in the relative proportions and absolute numbers of TcR gamma delta + cells, but an expansion of the recently described TcR beta + alpha - population also has a role. The expansion of the TcR gamma delta + population is polyclonal, as evidenced by the usage of multiple gamma and delta variable chain segments. Furthermore, a substantial proportion of the cells appears to be activated and these activated cells express surface activation markers. The results clearly demonstrate that TcR gamma delta + cells proliferate independently in response to a broad spectrum of challenges. Moreover, since the expansion of the lymphoid tissues and the TcR gamma delta + cell population is excessive relative to that seen in wild-type animals, one role of TcR alpha beta + cells is directly or indirectly to limit the responses of the other lymphoid components.

Viral induction of co-stimulatory activity on antigen-presenting cells bypasses the need for CD4+ T-cell help in CD8+ T-cell responses

BACKGROUND

CD4+ T-cell help is critical for cytotoxic (CD8+) T-lymphocyte responses to many antigens, such as viruses, minor histocompatibility antigens and allogeneic major histocompatibility antigens. However, the nature of such help is still a mystery: cytokines such as interleukin-2 may be involved but cell-cell contact may also be necessary. As some viruses can induce CD8+ T-cell responses in the absence of CD4+ T cells, we asked whether these viruses and CD4+ cells share a pathway for helping the CD8+ T-cell response.

RESULTS

We show here that the H2N2 subtype of influenza virus, which elicits a CD4+ T-cell-independent anti-viral CD8+ T-cell response in vitro, induces expression of the co-stimulatory molecule B7-2, but not of B7, on the cell surface of antigen-presenting cells. In contrast, the H1N1 subtype of influenza virus, which requires CD4+ T-cell help to elicit CD8+ T-cell responses under the same conditions, does not induce B7-2 expression. We also find that CD4+ T cells can induce expression of B7-2 on antigen-presenting cells. In both cases, the induced B7-2 is necessary for the clonal expansion and functional maturation of CD8+ T cells.

CONCLUSION

Our results support the view that the induction of co-stimulatory activity on antigen-presenting cells by CD4+ T cells can substitute for the requirement for exogenous interleukin-2 in CD8+ T-cell help. Viruses that can induce co-stimulatory activity on antigen-presenting cells thus induce a CD4+ T-cell-independent CD8+ T-cell response. These findings could explain the reported differences in the requirements for CD4+ T cells in CD8+ T-cell responses.

Epitopes on the beta subunit of human muscle acetylcholine receptor recognized by CD4+ cells of myasthenia gravis patients and healthy subjects

We investigated the sequence regions of the human muscle acetylcholine receptor (AChR) beta subunit forming epitopes recognized by T helper cells in myasthenia gravis (MG), using overlapping synthetic peptides, 20 residues long, which screened the sequence of the AChR beta subunit. Since CD4+ lymphocytes from MG patients' blood did not respond to the peptides, we attempted propagation of beta subunit-specific T lines from six MG patients and seven healthy controls by cycles of stimulation of blood lymphocytes with the pooled peptides corresponding to the beta subunit sequence. CD4+ T lines were obtained from four patients and three controls. They secreted IL-2, not IL-4, suggesting that they comprised T helper type 1 cells. The T lines from MG patients could be propagated for several months. Three lines were tested with purified bovine muscle AChR and cross-reacted well with this antigen. All T lines were tested with the individual synthetic peptides present in the pool corresponding to the beta subunit sequence. Several beta subunit peptide sequences were recognized. Each line had an individual pattern of peptides recognition, but three sequence regions (peptides beta 181-200, beta 271-290, and the overlapping peptides beta 316-335 and beta 331-350) were recognized by most MG lines. The beta subunit-specific T lines from controls could be propagated for < 5 wk. Each line recognized several peptides, which frequently included the immunodominant regions listed above.

Tumor immunity as autoimmunity: tumor antigens include normal self proteins which stimulate anergic peripheral T cells

Oncogenic transformation is often accompanied by a significant overproduction of otherwise normal cellular proteins. Because of the low expression of such self components during thymic development, T cells specific for these antigens can escape deletion and progress to the periphery. In this viewpoint, Giorgio Parmiani discusses the concept that these T cells can be harnessed to provide an effective anti-tumor response.

In vivo administration of major histocompatibility complex class I-specific peptides from influenza virus induces specific cytotoxic T cell hyporesponsiveness

We have been investigating the immunogenicity of two class I major histocompatibility complex-specific peptides with a sequence derived from influenza virus nucleoprotein specific for Kd and one for Db. Peptide-modified splenocytes are unable to immunize for a primary cytotoxic T (Tc) cell response in vivo, or secondary response in vitro. Peptide-modified stimulator cells can boost virus-primed splenocytes for a strong secondary response in vitro. Animals primed with syngeneic peptide-modified splenocytes upon challenge with virus in vivo do not generate strong secondary Tc cell responses on day 3 after challenge in contrast to virus primed animals. Day 6 responses of virus-challenged, peptide-primed animals are reduced as compared to unprimed mice. This hyporesponsiveness is independent of CD8+ T cells in the priming population and can be elicited with tumor cell lines. The data are discussed in the framework of the two-signal model of immune induction.

CD4+8- help prevents rapid deletion of CD8+ cells after a transient response to antigen

We have followed the fate of mature CD8+ T cells with a male-specific transgenic T cell receptor after antigenic stimulation with hemopoietic cells in the absence or presence of help. Our data show that mature CD8+ T cells can be deleted after a 3-week period of transient activation and that help, e.g. in the form of interleukin-2, can considerably delay the deletion. These experiments have implications for the design of protocols aiming at the establishment of specific immunological tolerance in T cells.

Helper activity of CD4+ alpha beta T cells is required for the avian gamma delta T cell response

We have studied the in vitro activation of chicken gamma delta T cells. Both splenic alpha beta and gamma delta T cells obtained from complete Freund's adjuvant-primed chickens proliferated in vitro when stimulated with mycobacterial sonicate or purified protein derivative of Mycobacterium tuberculosis. When CD4+ cells or alpha beta T cell receptor (TcR)-positive cells were removed, both the proliferation and the blast formation of gamma delta T cells in response to mycobacterial antigens were abrogated. The response was restored if supernatant from concanavalin A (Con A)-activated lymphocyte cultures (CAS) as a source of helper factors was added together with the specific antigen purified protein derivative. The CD4- or alpha beta TcR-depleted cells still proliferated in response to Con A, although a decrease of the response was observed. To analyze the gamma delta T cell response more specifically we stimulated peripheral blood cells with immobilized monoclonal antibodies against T cell receptor. Anti-gamma delta TcR antibody alone did not induce significant proliferation. When CAS was added together with the anti-gamma delta TcR monoclonal antibody, a strong proliferation of gamma delta T cells was observed. In contrast, both V beta 1- and V beta 2-expressing alpha beta T cells proliferated in vitro in response to stimulation with the relevant anti-TcR monoclonal antibody alone. Depletion of either V beta 1+ or V beta 2+ T cell subset alone had no negative effect on the proliferation or blast formation of gamma delta T cells stimulated with mycobacterial antigens. Taken together our results suggest that CD4+ alpha beta T cells (both V beta 1- and V beta 2-expressing) play a role in the activation and response of chicken gamma delta T cells.

Compartmentalization of T lymphocytes to the site of disease: intrahepatic CD4+ T cells specific for the protein NS4 of hepatitis C virus in patients with chronic hepatitis C

The adult liver is an organ without constitutive lymphoid components. Therefore, any intrahepatic T cell found in chronic hepatitis should have migrated to the liver after infection and inflammation. Because of the little information available on the differences between intrahepatic and peripheral T cells, we used recombinant proteins of the hepatitis C virus (HCV) to establish specific T cell lines and clones from liver biopsies of patients with chronic hepatitis C and compared them with those present in peripheral blood mononuclear cells (PBMC). We found that the protein nonstructural 4 (NS4) was able to stimulate CD4+ T cells isolated from liver biopsies, whereas with all the other HCV proteins we consistently failed to establish liver-derived T cell lines from 16 biopsies. We then compared NS4-specific T cell clones obtained on the same day from PBMC and liver of the same patient. We found that the 22 PBMC-derived T cell clones represent, at least, six distinct clonal populations that differ in major histocompatibility complex restriction and response to superantigens, whereas the 27 liver-derived T cell clones appear all identical, as further confirmed by cloning and sequencing of the T cell receptor (TCR) variable and hypervariable regions. Remarkably, none of the PBMC-derived clones has a TCR identical to the liver-derived clone, and even with polymerase chain reaction oligotyping we did not find the liver-derived clonotypic TCR transcript in the PBMC, indicating a preferential intrahepatic localization of these T cells. Functionally, the liver-derived T cells provided help for polyclonal immunoglobulin (Ig)A production by B cells in vitro that is 10-fold more effective than that provided by the PBMC-derived clones, whereas there is no difference in the help provided for IgM and IgG production. Altogether these results demonstrate that the protein NS4 is highly immunogenic for intrahepatic CD4+ T cells primed by HCV in vivo, and that there can be compartmentalization of some NS4-specific CD4+ T cells to the liver of patients with chronic hepatitis C.

In vivo priming effect during various stages of ontogeny of an influenza A virus nucleoprotein peptide

In vivo priming of cytotoxic T lymphocytes (CTL) by an influenza virus nucleoprotein (NP) peptide was studied at various stages of development. Adult mice immunized twice with the NP peptide in complete Freund's adjuvant and incomplete Freund's adjuvant, respectively, produce significant CTL responses. Neonates immunized at birth with large amounts of NP peptide and boosted twice with the peptide during adulthood, also mount a weak but significant CTL response. By contrast, offspring from mothers immunized with the NP peptide at days 15, 17, and 19 of pregnancy showed unresponsiveness to the peptide subsequent to a similar regimen of peptide immunization at the age of 1 month. The data indicate that the contact of T cell precursors with antigen during fetal life induces CTL tolerance, whereas, after birth the precursors are not susceptible to tolerogenic signals.

Induction of long-term H-Y-specific tolerance in female mice given male lymphoid cells while transiently depleted of CD4+ or CD8+ T cells

Rejection of H-Y-bearing primary skin grafts and generation of H-Y-specific cytolytic T cells by female mice requires the participation of both CD4+ and CD8+ T lymphocytes. Studies were conducted to investigate long-term tolerance of H-Y antigen induced in female mice by transiently depleting them of CD4+ and/or CD8+ T cells and, at the same time, giving them an injection of male lymphoid cells. We confirmed that after recovery of CD4+ to normal levels, female mice that had been transiently depleted of CD4+ cells and concurrently given an injection of male spleen cells were unable to generate H-Y-specific cytolytic T cells. Tolerance was also manifest by greatly extended survival (probably permanent in most cases) of male skin grafts. Further investigations revealed that female mice transiently depleted of CD8+ cells, and concurrently given an injection of male spleen cells, were similarly tolerant of H-Y antigen later when numbers of CD8+ T cells returned to normal. Moreover, small numbers of male cells were detectable in spleen and lymph nodes of tolerant females long after they had been given an injection of male cells and depleted of either CD4+ or CD8+ T cells, whereas no male cells were detected in (nontolerant) females given male cells and control antibodies. These findings show that tolerance of the relatively weak transplantation antigen, H-Y, can be achieved simply by giving male antigen-bearing spleen cells to the host while it is transiently depleted of a type of cell it needs in order to reject those cells, thus allowing the male cells to persist in the host. Furthermore, depletion of helper cells is not obligatory to achieve tolerance. It has been hypothesized that tolerance of H-Y antigen in females given male lymphoid cells while temporarily depleted of CD4+ lymphocytes results from unresponsiveness (anergy) induced in H-Y-specific CD8+ cells that are exposed to H-Y antigen in the absence of help from CD4+ cells. Interpretations of our findings are discussed in relation to this hypothesis.

Thymus epithelium induces tissue-specific tolerance

Most current models of T cell development include a positive selection step in the thymus that occurs when T cells interact with thymic epithelium and a negative selection step after encounters with bone marrow-derived cells. We show here that developing T cells are tolerized when they recognize antigens expressed by thymic epithelium, that the tolerance is tissue specific, and that it can occur by deletion of the reactive T cells.

B cells turn off virgin but not memory T cells

There are three possible outcomes when a T cell recognizes a cell bearing a self or foreign antigen. (i) The T cell is not sufficiently signaled and is unaffected. (ii) The T cell is activated. (iii) The T cell is turned off. The differentiation state of the T cell is critical to the outcome. Although both virgin and memory T cells can be activated by antigens presented by "professional" antigen-presenting cells such as dendritic cells, they differ in their responses to B cells. Experienced T cells respond to antigen presented by B cells, whereas virgin T cells are rendered tolerant. These findings may relate to the phenomena of low- and high-zone tolerance, neonatal tolerance, and the beneficial effect of blood transfusions on allograft survival.