Expression of L-selectin (CD62L) discriminates Th1- and Th2-like cytokine-producing memory CD4+ T cells.

Human memory (CD45RO+) CD4+ T cells can be distinguished into two subpopulations on the basis of expression of the lymph node homing receptor, L-selectin (CD62L). In a prior study we showed that human L-selectin-positive memory T-helper (Th) cells promote the maturation of IgG- and IgA-producing cells by naive B cells. To further elucidate the contribution of memory CD4+ T cells to B-cell differentiation, human memory CD4+ T cells with or without L-selectin expression were evaluated for production of cytokines that participate in regulation of immunoglobulin production. It was found that L-selectin-positive human memory CD4+ T cells produce mainly interleukin (IL)-4 and IL-5, whereas L-selectin-negative CD4+ T cells produce mainly interferon-gamma (IFN-gamma). This profile of cytokine expression coincides with the profile that distinguishes Th1 and Th2 subsets. In contrast to the murine system, IL-10 production was similarly contributed by human L-selectin-positive and -negative memory CD4+ T-cell subpopulations. These results suggest that the human L-selectin-negative and -positive subpopulations of human memory CD4+ T cells contain Th1-like and Th2-like cytokine-producing cells, respectively.     

IL-2抑制小鼠CD4~+ CD62L~+ T细胞分化为Th17细胞

目的研究IL-2对小鼠脾脏CD4+CD62L+T细胞在体外向Th17细胞分化的作用。方法免疫磁珠法分选C57BL/6小鼠脾脏CD4+CD62L+T细胞,于抗体包被的培养板中培养3 d,实验分为对照组和IL-2处理组。对照组为经典Th17诱导分化培养基,IL-2处理组在对照组基础上于培养体系中添加IL-2。CFSE染色检测细胞增殖,Annexin V-PI法检测细胞凋亡,ELISA检测培养上清中IL-17A的浓度,荧光定量PCR检测Rorγt mRNA的表达,流式细胞术检测CD4~+IL-17~+Th17的生成比例以及Rorγt的表达。结果磁珠分选的小鼠脾脏CD4+CD62L+nave T细胞纯度高于95%。与对照组相比,IL-2处理组细胞数目明显增多,增殖能力明显增强(P0.05),细胞凋亡比例降低(P0.05);IL-2处理组培养上清中IL-17A的浓度明显降低(P0.05),且CD4+IL-17+细胞比例下降,其特异性转录因子Rorγt的表达水平也显著降低(P0.05)。结论 IL-2在CD4+CD62L+T细胞分化为Th17的过程中,能够促进T细胞的增殖并且抑制Th17的分化。     

Altered Th1/Th2 commitment in human CD4+ T cells with ageing

The human immune system undergoes continuous remodelling with the advancement of age. Since age-associated functional alterations in the immune system could be caused by a possible change in helper T cell regulation in elderly subjects, we comparatively studied the function of CD4+ T cells in peripheral blood obtained from both young and old healthy volunteers. Upon cell activation by phorbol myristate acetate and ionomycin, the proportion of CD4+ T cells containing interferon-gamma (IFN-gamma) was found to be greater in the old subjects. Utilizing a co-culture system, which activated CD4+ T cells via the TCR/CD3 complex and CD28, we found that CD4+ T cells from the old subjects secreted more IFN-gamma and IL-2, but less IL-4, than those from the young subjects. Upon cell activation by co-culture, CD4+ T cells from the old subjects expressed more CD26, CD40L, and LFA-1, but less CD30, than those from the young. These results together suggest that the microenvironment in which CD4+ T cells develop in older people may cause production of more cells committed to Th1 than that in younger subjects.     

Differential regulation of Th1 responses and CD154 expression in human CD4+ T cells by IFN-alpha

Like interleukin (IL)-12, interferon (IFN)-alpha has been shown to play an important role in inducing human Th1 responses. Recent studies have shown that human Th1 responses driven by IL-12 are associated with enhanced expression of CD154. The present study examined the effects of IFN-alpha on CD154 expression in human CD4+ T cells, with special attention to the relationship with Th1 responses. Highly purified CD4+ T cells from healthy donors were stimulated with immobilized anti-CD3 with or without IFN-alpha and IL-12 in the complete absence of accessory cells. IFN-alpha suppressed CD154 protein and mRNA expression in CD4+ T cells at the initial phase of activation with immobilized anti-CD3, but enhanced it in the subsequent maturation phase irrespective of the presence of IL-12. By contrast, IFN-alpha by itself did not enhance IFN-gamma production or mRNA expression in CD4+ T cells in the absence of IL-12 even in the presence of stimulation with anti-CD28, but enhanced it in the presence of IL-12. Accordingly, IFN-alpha enhanced IL-12Rbeta2 mRNA expression in anti-CD3-stimulated CD4+ T cells. Neither IFN-alpha nor IL-12 influenced the stability of CD154 mRNA in anti-CD3-activated CD4+ T cells. These results indicate that IFN-alpha by itself enhances CD154 expression in CD4+ T cells independently of the induction of IFN-gamma mRNA expression. The data also suggest that the optimal induction of human Th1 responses by IFN-alpha might require the presence of IL-12 and that the induction of Th1 responses and CD154 expression in human CD4+ T cells might be regulated through different mechanisms.     

Differential regulation of IL-13 and IL-4 production by human CD8+ and CD4+ Th0, Th1 and Th2 T cell clones and EBV-transformed B cells

In the present study, the requirements and characteristics forthe production of IL-13 by human T cells, T cell clones andB cells were determined and compared with those of IL-4. IL-13was produced by human CD4⁺ and CD8⁺ T lymphocyte subsets isolatedfrom peripheral blood mononuclear cells and by CD4⁺ and CD8⁺T cell clones. CD4⁺ T cell clones belonging to T_h0, T_h1-likeand T_h2-like subsets produced IL-13 following antigen-specificor polyclonal activation. In addition, EBV-transformed B celllines expressed IL-13 mRNA and produced small amounts of IL-13protein. Expression of IL-13 mRNA and production of IL-13 proteinby peripheral blood T cells and T cell clones was induced rapidlyand was relatively long lasting, whereas IL-4 production bythese cells was transient In addition, IL-13 mRNA expressionwas induced by modes of activation that failed to induce IL-4mRNA expression. IL-13 shares many biological activities withIL-4 which Is compatible with the notion that the IL-13 andIL-4 receptors share a common component required for signaltransduction. However, IL-13 lacks the T cell-activating propertiesof IL-4. Here we have shown that this is related to the factthat T cells fall to bind radiolabeled IL-13 and do not expressthe IL-13-speclflc receptor component Taken together, theseresults indicate that the differences In expression and biologicalactivities of IL-4 and IL-13 on T cells may have consequencesfor the relative roles of these cytokines In the immune response.     

Interleukin-11 modulates Th1/Th2 cytokine production from activated CD4+ T cells.

Recombinant human interleukin-11 (rHuIL-11) is a pleiotropic cytokine with effects on multiple cell types. rHuIL-11 reduces activated macrophage activity and downregulates production of proinflammatory mediators, such as tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO). In vitro and in vivo, rHuIL-11 inhibits production of key immunostimulatory cytokines, including IL-12 and interferon-gamma (IFN-gamma). rHuIL-11 has recently demonstrated immunomodulatory activity to downregulate IFN-gamma production, increase IL-4 production, and reduce inflammatory tissue injury in a human psoriasis clinical trial. The cellular mechanisms of these effects are not fully elucidated. We demonstrate here that expression of gp130 and IL-11 receptor (IL-11R) alpha mRNA, components of the IL-11R complex, are detected in human and murine CD4(+) and CD8(+) lymphocytes, suggesting that rHuIL-11 can directly interact with T cells. In a cell culture model of murine T cell differentiation, rHuIL-11 acts to inhibit IL-2 production as well as IL-12-induced IFN-gamma production and enhances IL-4 and IL-10 production. rHuIL-11 had no effect on T cell proliferation. The ability of rHuIL-11 to modulate cytokine production from activated CD4(+) T cells provides a mechanism through which rHuIL-11 may ameliorate such inflammatory diseases as psoriasis.     

Organ-specific distribution of CD4+ T1/ST2+ Th2 cells in Leishmania major infection

Activated CD4(+) T helper cells (Th) comprise at least two functionally distinct subsets, Th1 and Th2, which mediate different immunological effector functions. Experimental leishmaniasis is widely used to study the effector function of Th cell subsets in vivo. Healing and nonhealing Leishmania major infections have been correlated with polarized Th1 and Th2 responses, respectively. In the study presented here, a stable cell surface marker expressed on Th2 cells, T1/ST2, has been used to assess the distribution of CD4(+) T1/ST2(+) T cells in different organs of healer and nonhealer strains of mice during the course of L. major infection. The frequency of CD4(+) T cells expressing the T1/ST2 cell surface marker and Th2 cytokines in the lymphoid organs was low in both strains of infected mice; however, CD4(+) T1/ST2(+) T cells could be enriched from the lymphoid organs of infected nonhealer but not from healer strains of mice. The highest frequency of CD4(+) T1/ST2(+) T cells was detected in the footpads of mice with nonhealing disease, showing that CD4(+) T1/ST2(+) T cells home to the footpads. Since the majority of parasites persist at the local site of infection in nonhealing BALB/c mice, these results show that CD4(+) T1/ST2(+) T cells are localized at the site of active infection and inflammation in this model.     

Immunomodulation of human CD19+CD25high regulatory B cells via Th17/Foxp3 regulatory T cells and Th1/Th2 cytokines

Regulatory B (Breg) cells are a special subset of immunoregulatory cells with unique phenotypes and functions. In this study, human CD19⁺CD25^high Breg cells were purified from human peripheral blood. Based on the coculture system of Breg cells and CD4⁺ T cells in vitro, Breg cells were found to promote the increase in regulatory T (Treg) cells while decreasing the number of Th17 cells. Breg cells regulate Treg cells through two processes: cell-cell contact and cytokines. TGF-βsRII, a blocker of transforming growth factor-β (TGF-β), can attenuate the effects of Treg elevation, suggesting that TGF-β is the main cytokine, while Breg cells rather than interleukin-10 (IL-10) regulate the differentiation of Treg cells. However, Th17 cells were mainly regulated by cytokines, without an obvious regulatory effect on cell-cell contacts. Breg cells may regulate Th17 cells by a pathway independent of TGF-β and IL-6. The coculture of Breg cells and CD4⁺ T cells led to changes in the cytokine spectrum, which included significant increases in IL-4, IL-6 and IL-10 but not obvious changes in IL-2, IFN-γ and TNF. The inhibitory effect of Breg cells was weakened by blocking cell-cell contacts in cultures separated with the Transwell chamber because IL-10 decreased while IL-6 increased when compared with cocultured Breg and CD4⁺ T cells. When the IL-10 inhibitor IL-10sRα was added, IL-6 and TNF levels significantly increased, while treatment with the TGF-β inhibitor TGF-βsRII did not result in similar changes, suggesting that IL-10 is an important molecule to inhibit the proinflammatory factors IL-6 and TNF in this culture system.     

Th1 and Th2 CD4+ T cells in the pathogenesis of organ-specific autoimmune diseases

CD4⁺ T cells play a key role in regulating immune system function. When these regulatory processes go awry, organ-specific autoimmune diseases may develop. Here, Roland Liblau, Steven Singer and Hugh McDevitt explore the thesis that a particular subset of CD4⁺ T cells, namely T helper 1 (Th1) cells, contributes to the pathogenesis of organ-specifc autoimmune diseases, while another subset, Th2 cells, prevents them.     

Modulation of monocyte/macrophage function by human CD4+CD25+ regulatory T cells

The suppressive effects of CD4+CD25+ regulatory T cells (Tregs) on T cells have been well documented. Here we investigated whether human CD4+CD25+ Tregs can inhibit the proinflammatory properties of monocytes/macrophages. Monocytes and T cells were isolated from peripheral blood of healthy volunteers by magnetic cell separation and cocultured for 40 h. Monocytes were analyzed directly for cytokine production and phenotypic changes or repurified and used in T-cell stimulation and lipopolysaccharide challenge assays. Coculture with CD4+CD25+ Tregs induced minimal cytokine production in monocytes, whereas coculture with CD4+CD25- T cells resulted in large amounts of proinflammatory (tumor necrosis factor-alpha, interferon-gamma, interleukin-6) and regulatory (interleukin-10) cytokines. Importantly, when these CD4+CD25+ Treg-treated monocytes were repurified after coculture and challenged with lipopolysaccharide, they were severely inhibited in their capacity to produce tumor necrosis factor-alpha and interleukin-6 compared with control-treated monocytes. In addition, monocytes that were precultured with CD4+CD25+ Tregs displayed limited upregulation of human leukocyte antigen class II, CD40 and CD80, and downregulation of CD86 compared with control-treated monocytes. This altered phenotype had functional consequences, as shown by the reduction in T cell-stimulatory capacity of Treg-treated monocytes. Together, these data demonstrate that CD4+CD25+ Tregs can exert direct suppressive effects on monocytes/macrophages, thereby affecting subsequent innate and adaptive immune responses.     

CD4+ T cells and the Th1/Th2 imbalance are implicated in the pathogenesis of Graves' ophthalmopathy

Graves' ophthalmopathy (GO) is considered to be an organ-specific autoimmune disease. However, the pathogenesis of GO is incompletely understood at the present time. To clarify the immunological differences between newly diagnosed GO and Graves' disease (GD) without ophthalmopathy or healthy controls (HC), we examined T-cell profile and the Th1/Th2 profile cell balance in GO (n=20), GD (n=20) and HC (n=20) using flow cytometry. We also assessed the influence of methimazole on the immunocyte profiles in patients with GO and GD and analyzed the relationship of the immunologic changes with CAS, FT3, FT4, TRAb, TMA and TGA among the three investigated groups. We report in this study that: 1) The percentage of CD4+ T cells and the ratio of CD4+/CD8+ cells were higher, but the population of CD8+ T cells was lower in both GO and GD than those of HC (P<0.05); 2) The percentage of CD8-/IFNgamma+ T cells (Th1) and the ratio of CD8-/IFNgamma+ to CD8-/IL-4+ T cells (Th1/Th2) in GO were considerably higher as compared to those in GD and HC (P<0.05). On the contrary, the population of Th1 cells, as well as the ratio of Th1/Th2 cells, was lower in GD than that of GO and HC (P<0.05); 3) There were no significant differences in T-cell profile and the Th1/Th2 cell balance in either GO or GD patients before and after methimazole treatment; 4) There was a positive correlation of Th1 cell percentage and the Th1/Th2 cell ratio with the clinical activity score (CAS) in GO (P<0.05), whereas CAS in GO had no correlation with the T-cell profile, the percentage of Th2 cells, and TRAb (P>0.05); 5) T-cell subset and the ratio of Th1/Th2 cells did not correlate significantly with FT3, FT4, TRAb, TMA, or TGA in GO and GD (P>0.05). Finally, 6) there were no statistical differences in TRAb, TMA, and TGA between early GO and GD without ophthalmopathy (P>0.05). Collectively, these results indicate that the balance of Th1/Th2 in GO shifts to Th1 dominance and that the cellular immune responses mediated by the Th1-type CD4+ cells might play a dominant role in the pathogenesis of GO, and thus suggest that the Th1 cell percentage and the ratio of Th1/Th2 cell subsets may be potentially utilized as clinical parameters for disease activity, for monitoring the effectiveness of immunosuppressive treatment, or for developing immunospecific forms of therapy for Graves' ophthalmopathy.     

4-1BB-dependent inhibition of immunosuppression by activated CD4+CD25+ T cells

4-1BB (CD137) is a costimulatory molecule involved in the activation and survival of CD4, CD8, and natural killer cells. Although a great deal has been learned as to how 4-1BB-mediated signaling governs the immunity of conventional T cells, the functional role of 4-1BB in the context of CD4(+)CD25(+) regulatory T cell (Tr) activation is largely unknown. Using 4-1BB-intact and -deficient mice, we investigated the effect of the 4-1BB/4-1BB ligand pathway on the suppressive function of Tr cells. Our data indicate that although 4-1BB is expressed on Tr cells, its contribution to their proliferation is minimal. We also showed that signaling through the 4-1BB receptor inhibited the suppressive function of Tr cells in vitro and in vivo. It is interesting that anti-4-1BB-mediated but not anti-GITR-directed inhibition was more potent when Tr cells were preactivated. Collectively, these data indicate that 4-1BB signaling is critical in Tr cell immunity.     

T helper 1 (Th1)/Th2 cytokine expression shift of peripheral blood CD4+ and CD8+ T cells in patients at the post-acute phase of stroke

Local humoral and cellular immune responses modulate the inflammatory processes involved in the development of atherosclerotic lesions, as well as in the evolution of brain infarcts in stroke patients. The role of systemic adaptive immunity on the progression of such disease manifestations is less clear. In the current study, we evaluated the percentages of T helper 1 (Th1) [interleukin (IL)-2, interferon (IFN)-gamma] and Th2 (IL-4, IL-10) cytokine-producing peripheral blood CD4+ and CD8+ T cells in 23 patients with a history of ischaemic stroke (IS) at the chronic stable phase of the disease (median post-stroke time 34.5 months). Seven stroke-free individuals matched for age and vascular risk factors (matched controls, MC) were collected for comparison. To measure cytokine values at baseline and after stimulation, we used a flow cytometry method of intracellular cytokine staining. Intrinsic Th1 and Th2 cytokine production in unstimulated T cells was negligible in all study participants. Following mitogenic stimulation with phorbol 12-myristate13-acetate/ionomycin, both the IS and the MC groups exhibited a similarly strong Th1 response; IL-2 production predominated in the CD4+ T cells and IFN-gamma in the CD8+ T cells. However, when measuring the Th2 cytokine-production capacity post-stimulation, a significant increase in the percentage of IL-4-producing T cells was observed in the IS groups, compared with the MC group, resulting in a significantly lower ratio of IFN-gamma-/IL-4-producing T cells. No such Th2 enhancement could be confirmed for the case of IL-10. We propose that in IS patients there is a systemic shift of the immune system towards Th2 responses at the late post-acute phase of stroke.     

大鼠CD4+ CD25+ T调节细胞的分离培养及其功能分析

目的:研究大鼠CD4 CD25 T调节细胞(Tr)的分离培养,并对其功能进行初步分析。方法:无菌条件下切取大鼠脾脏分离脾淋巴细胞。用免疫磁珠细胞分离系统(MACS)分选CD4 CD25 T细胞,并以流式细胞术检测其纯度后,对其进行扩增。采用混合淋巴细胞反应研究CD4 CD25 Tr细胞对CD4 CD25-T细胞的免疫抑制作用。用ELISA法检测培养上清中IL-2、IFN-γ及IL-10水平的差异。结果:MACS分离的CD4 CD25 T细胞的纯度达86%~93%。该细胞与CD4 CD25-T细胞相比能特异性地表达Foxp3基因。体外培养中能明显抑制效应T细胞增殖及其分泌IFN-γ、IL-2,但其自身能分泌Th2型细胞因子IL-10。结论:采用MACS系统阴性加阳性分选,可高效快速的获得理想纯度和免疫抑制功能的大鼠CD4 CD25 T调节细胞,该细胞对CD4 CD25-T细胞具有明显的免疫抑制作用,并能特异性的表达Foxp3基因。     

小鼠CD4+CD25+T细胞的分离、鉴定和体外功能的检测

目的　建立CD4 CD2 5 T细胞的免疫磁性分离(MACS)方法,并检测其体外免疫抑制功能。方法　分离C5 7BL 6小鼠和Balb c小鼠脾脏细胞后运用MACS法分离CD4 CD2 5 T淋巴细胞,用台盼蓝染色法和流式细胞法检测选出细胞的活性和纯度,用淋巴细胞转化实验分析其免疫抑制功能。结果　MACS分选法分选出的CD4 CD2 5 T细胞活性>97% ,纯度>95 % ;此方法获得的CD4 CD2 5 T细胞在体外不增殖,经丝裂原活化后可以抑制同品系小鼠和异品系小鼠的CD4 CD2 5 -T细胞增殖,这种抑制作用具有剂量依赖性。结论　MACS法可简单迅速分选出高纯度无菌CD4 CD2 5 T细胞,且不影响细胞活力。CD4 CD2 5 T调节性细胞在体外具有免疫无能和免疫抑制作用;丝裂原活化的该细胞免疫抑制功能具有剂量依赖性,但不具有抗原特异性。     

CD28 activation promotes Th2 subset differentiation by human CD4+ cells

Ligation of CD28 provides a costimulatory signal to T cells necessary for their activation resulting in increased interleukin (IL)-2 production in vitro, but its role in IL-4 and other cytokine production and functional differentiation of T helper (Th) cells remains uncertain. We studied the pattern of cytokine production by highly purified human adult and neonatal CD4⁺ T cells activated with anti-CD3, phorbol 12-myristate 13-acetate (PMA) and ionomycin, or phytohemagglutinin (PHA) in the presence or absence of anti-CD28 in repetitive stimulation-rest cycles. Initial stimulation of CD4⁺ cells with anti-CD3 (or the mitogens PHA or PMA+ionomycin) and anti-CD28 monoclonal antibodies induced IL-4, IL-5 and interferon-γ (IFN-γ) production and augmented IL-2 production (6- to 11-fold) compared to cells stimulated with anti-CD3 or mitogen alone. The anti-CD28-induced cytokine production corresponded with augmented IL-4 and IL-5 mRNA levels suggesting increased gene expression and/or mRNA stabilization. Most striking, however, was the progressively enhanced IL-4 and IL-5 production and diminished IL-2 and IFN-γ production with repetitive consecutive cycles of CD28 stimulation. The enhanced Th2-like response correlated with an increased frequency of IL-4-secreting cells; up to 70% of the cells produced IL-4 on the third round of stimulation compared to only 5% after the first stimulation as determined by ELISPOT. CD28 activation also promoted a Th2 response in naive neonatal CD4⁺ cells, indicating that Th cells are induced to express a Th2 response rather than preferential expansion of already established Th2-type cells. This CD28-mediated response was IL-4 independent, since enhanced IL-5 production with repetitive stimulation cycles was not affected in the presence of neutralizing anti-IL-4 antibodies. These results indicate that CD28 activation may play an important role in the differentiation of the Th2 subset in humans.     

Marked anti-tumor effects of CD8+CD62L+ T cells from melanoma-bearing mice

CD8⁺CD62L⁺ T cells have been shown to play pivotal roles in anti-viral immunity, chronic myeloid leukemia and renal cell carcinoma. Recently, CD8⁺CD62L⁺ T cells from naïve mice (nCD8⁺CD62L⁺ T cells) have shown superior anti-tumor properties in melanoma-bearing mice. Considering that antigen-specific memory T cells have shown to possess more potent immunity than non-specific memory T cells, we hypothesized that CD8⁺CD62L⁺ T cells from tumor-bearing individuals (mCD8⁺CD62L⁺ T cells) might have superior anti-tumor effect than nCD8⁺CD62L⁺ T cells. Therefore, we investigated phenotypes, functions and the in vivo distribution of mCD8⁺CD62L⁺ T cells in tumor-bearing mice. We found that, while keeping the features of central memory T cells, the frequency of mCD8⁺CD62L⁺ T cell in the spleen of tumor-bearing mice was significantly higher than that the one of nCD8⁺CD62L⁺ T cell in naive mice. Moreover, we demonstrated that mCD8⁺CD62L⁺ T cells had higher proliferation rate and IFN-γ production than nCD8⁺CD62L⁺ T cells, in vitro. We performed adoptive transfer of mCD8⁺CD62L⁺ T cells into melanoma-bearing mice and tracked them in spleen, lymph nodes and in melanoma tissues. Our results show that mCD8⁺CD62L⁺ T cells had stronger in vivo anti-tumoral activity than nCD8⁺CD62L⁺ T cells. This study highlights the therapeutic potential of mCD8⁺CD62L⁺ T cells in the immunotherapy of melanoma and possibly other tumors.     

T cell receptor specificities of Toxoplasma gondii-reactive mouse CD4+ T lymphocytes and Th1 clones

The Toxoplasma gondii-directed CD4⁺ T cell response in chronically infected mice was studied with respect to both T cell receptor diversity and antigen specificities. T cell receptor chains Vβ4, 6, 8, 10, and 14 were predominantly found on toxoplasma-reactive CD4⁺ splenocytes. This repertoire was also detected among T. gondii-specific CD4⁺ T cell clones. Analysis of clonotypic cytokine profiles revealed typical Th1 clones secreting interleukin-2, interferon-γ and tumour necrosis factor activity and Th2 clones producing interleukin-4 and interleukin-10. Five distinct toxoplasma antigens (p26, p40, p55, p58 and p60) were detected in electrophoretically separated toxoplasma lysate by five individual Th1 clones. Parallel testing of CD4⁺ T lymphocytes from infected mice confirmed that these specificities constitute the peak immunogenic fractions of toxoplasma lysate. The expression patterns of two clonotypic, T cell-stimulatory parasite antigens were studied in detail. While p55 was expressed by mouse-virulent and avirulent T. gondii isolates and in both the tachyzoite and bradyzoite stages, p58 was detected only in virulent strains from intraspecies subgroup I. Thus, we describe the heterogeneity of toxoplasmic immunodominant T cell antigens including a 58-kDa group I-restricted molecule which may provide a marker for virulent isolates. Received: 3 February 1997