POS1421 AROMATASE INHIBITORS AND SKELETAL HEALTH – NATURAL HISTORY AND INTERVENTIONAL EPIDEMIOLOGY

Background

Breast cancer remains the most common cancer diagnosed in women worldwide. Aromatase inhibitors (AI) are employed for hormone sensitive disease in mainly postmenopausal women. AI related bone loss (AIBL) is a known complication; although data regarding the natural history in the real-world, long-term outcomes and the role of bone active therapy in fracture prevention is sparse.

Objectives

Our aim was to determine the real-world impact of AIBL and whether bone sparing therapy utilising standard risk stratification model is sufficient for fracture prevention.

Methods

We undertook a longitudinal study of patients prescribed AI for breast cancer over a seven-year period at our university teaching hospital. All the data was recorded electronically with full access to demographics, disease parameters, investigations and drug management. DEXA scans performed prior to initiation of AI were compared with subsequent imaging over a mean follow up of 3 years. Outcome data for cancer and all fractures was collected. Statistical analysis was done to investigate significant relationships amongst the variables of interest.

Results

1001 women were identified during the study period. The mean age of the cohort was 64 years (range 29-93). 929 (93%) were Caucasian, 57 (6%) were Asian and 15 (1%) were Afro-Caribbean. 723 women (72%) were diagnosed with invasive ductal carcinoma and 863 women (86%) were postmenopausal. At diagnosis, 428 women (43%) had node positive disease and 35 women (4%) had metastases. 91 women (9%) had sustained fractures prior to their breast cancer diagnosis.

All women had a baseline DEXA: 496 (49.6%) had osteopenia, 151 (15%) had osteoporosis and 354 (35.4%) had a normal result. 478 (48%) of women had a repeat scan available. Overall, there was a decline (from a mean of 0.888 g/cm2 to 0.858 g/cm2, p<0.0001) in left neck of femur (LNOF) bone mineral density (BMD) over time (mean of 3 years, with a range of 1-6).

334 (33%) were prescribed bone active therapy with 276 women (83%) given oral bisphosphonates. This group had an improvement in BMD by 0.4% (LNOF mean BMD of 0.785 g/cm2 at baseline compared to LNOF mean BMD of 0.788 at repeat DEXA, p=0.82).

Women who were not offered any treatment (n=667, 66%), showed a significant decline in bone density with the decline being -5%. (LNOF mean BMD of 0.939 g/cm2 at baseline compared to LNOF mean BMD of 0.888 g/cm2 at repeat DEXA, p< 0.0001).

The rate of fractures remained the same between the treatment (19 fractures, 5.67%) and non-treatment group (38 fractures, 5.70%)

Conclusion

Our study provides long term data for AIBL and confirms a significant decline in BMD over seven years. It confirms that bone sparing therapy is effective in reducing the pace of decline in BMD. However standard risk stratification model such as FRAX based intervention thresholds in mainly those with WHO defined osteoporosis (T ≤-2.5) is ineffective in fracture prevention in keeping with prior literature. Since our study period overlaps with publication of newer guidelines recommending different T score-based risk model, further studies are required to confirm their utility.

References

[1]https://www.wcrf.org/dietandcancer/worldwide-cancer-data/. Date accessed: 26.01.2022

[2]R. Coleman, J.J. Body, M. Aapro, et al., Bone health in cancer patients: ESMO clinical practice guidelines, Ann. Oncol. 25 (Suppl 3) (2014) iii124–iii137.

[3]E. Amir, B. Seruga, S. Niraula, et al., Toxicity of adjuvant endocrine therapy in postmenopausal breast cancer patients: a systematic review and meta-analysis, J. Natl. Cancer Inst. 103 (2011) 1299–1309.

Disclosure of Interests

None declared

OP0244 AROMATASE INHIBITORS AND FRACTURE PREVENTION – DO NEW GUIDELINES WORK IN REAL WORLD?

Background

2,261,419 women were diagnosed with breast cancer worldwide in 2020. For postmenopausal women with hormone sensitive disease, aromatase inhibitors (AI) are recommended for their mortality benefit. However, AI bone loss (AIBL) is a recognised adverse event with resultant increase in fracture risk. In 2017, a consensus statement of 7 international bone and cancer societies was published proposing an algorithm based on clinical risk factors and different bone mineral density (BMD) threshold for bone active therapeutic intervention.

Objectives

To determine the real-world impact of the 2017 consensus guidelines on AIBL and whether bone sparing therapy utilising proposed risk stratification model is effective in fracture prevention.

Methods

Over a 7-year study period, 1001 women were prescribed AI at our university teaching hospital. The new guidelines were adopted in July 2017. We split the participants in two groups: 361 (36%) women had commenced their AI prior to the adoption of guidelines and 640 (64%) were in the post implementation group.

First group were offered bone active treatment based on NOS 2009 guidelines whereas the second group followed the 2017 consensus guidelines. Women with osteoporosis were all offered treatment, however the difference in guideline is pertinent to osteopenia and we compared the results of that group.

Results

1001 women were included. Mean age was 64 years (range 29-93). 929 (93%) were Caucasian, 57 (6%) were Asian and 15 (1%) were Afro-Caribbean. 723 women (72%) had invasive ductal carcinoma and 863 women (86%) were postmenopausal. At diagnosis, 428 women (43%) had node positive disease and 35 women (4%) had metastases. 91 women (9%) had sustained fractures prior to their cancer diagnosis.

276 women (28%) were offered oral bisphosphonates based on DEXA result, with 58 (6%) offered parenteral therapy.

First group: 361 women had a baseline DEXA with a mean left neck of femur (LNOF) BMD of 0.888 g/cm2 (range 0.552-1.222). 143 (40%) women had a normal DEXA, 174 (48%) had osteopenia and 44 (12%) had osteoporosis.

Of the women with osteopenia, 44 (25%) women were offered treatment and 33 women had a repeat DEXA after a mean of 4 years. In the treatment group, LNOF mean BMD remained relatively unchanged from 0.814 g/cm2 to 0.812 g/cm2 at the repeat DEXA (p= 0.94).

Of the 174 women with osteopenia, 22 (13%) women had a fracture.

Second group: 640 women had a baseline DEXA with a mean LNOF BMD of 0.888 g/cm2 (range 0.512-1.390). 216 (33%) women were normal, 322(50%) had osteopenia and 107 (17%) had osteoporosis.

Of the women with osteopenia, 127 (39%) women were offered treatment and 56 women had a repeat DEXA after a mean of 3 years. In the treatment group, LNOF mean BMD remained relatively unchanged from 0.822 g/cm2 to 0.829 g/cm2 at the repeat DEXA (p= 0.6169).

Of the 322 women with osteopenia, 8 (2.5%) women had a fracture.

Conclusion

Our study provides real world evidence of the success of 2017 consensus statement in lowering fracture risk. Though there has been data for positive impact on BMD decline with this approach, evidence for fracture prevention has been limited. This study showcases the success of lowering bone active therapy threshold employing alternative risk modelling strategy for women with breast cancer commenced on AI. A significant reduction in fractures pre (13%) and post guidelines change (2.5%) was demonstrated (absolute risk reduction of 10.5%) which has implications for healthcare systems worldwide as we have demonstrated this approach can reduce morbidity.

References

[1]https://www.wcrf.org/dietandcancer/worldwide-cancer-data/. Accessed: 26.01.2022.

[2]Reid DM, Doughty J, Eastell R, et al. Guidance for the management of breast cancer treatment-induced bone loss: a consensus position statement from a UK Expert Group. Cancer Treat Rev. 2008;34 Suppl 1:S3-S18.

[3]Hadji P, Aapro MS, Body JJ, et al. Management of Aromatase Inhibitor-Associated Bone Loss (AIBL) in postmenopausal women with hormone sensitive breast cancer: Joint position statement of the IOF, CABS, ECTS, IEG, ESCEO IMS, and SIOG. J Bone Oncol. 2017;7:1-12.

Disclosure of Interests

None declared

From high to low malaria transmission in Zanzibar-challenges and opportunities to achieve elimination

BACKGROUND

Substantial global progress in the control of malaria in recent years has led to increased commitment to its potential elimination. Whether this is possible in high transmission areas of sub-Saharan Africa remains unclear. Zanzibar represents a unique case study of such attempt, where modern tools and strategies for malaria treatment and vector control have been deployed since 2003.

METHODS

We have studied temporal trends of comprehensive malariometric indices in two districts with over 100,000 inhabitants each. The analyses included triangulation of data from annual community-based cross-sectional surveys, health management information systems, vital registry and entomological sentinel surveys.

RESULTS

The interventions, with sustained high-community uptake, were temporally associated with a major malaria decline, most pronounced between 2004 and 2007 and followed by a sustained state of low transmission. In 2015, the Plasmodium falciparum community prevalence of 0.43% (95% CI 0.23-0.73) by microscopy or rapid diagnostic test represented 96% reduction compared with that in 2003. The P. falciparum and P. malariae prevalence by PCR was 1.8% (95% CI 1.3-2.3), and the annual P. falciparum incidence was estimated to 8 infections including 2.8 clinical episodes per 1000 inhabitants. The total parasite load decreased over 1000-fold (99.9%) between 2003 and 2015. The incidence of symptomatic malaria at health facilities decreased by 94% with a trend towards relatively higher incidence in age groups > 5 years, a more pronounced seasonality and with reported travel history to/from Tanzania mainland as a higher risk factor. All-cause mortality among children < 5 years decreased by 72% between 2002 and 2007 mainly following the introduction of artemisinin-based combination therapies whereas the main reduction in malaria incidence followed upon the vector control interventions from 2006. Human biting rates decreased by 98% with a major shift towards outdoor biting by Anopheles arabiensis.

CONCLUSIONS

Zanzibar provides new evidence of the feasibility of reaching uniquely significant and sustainable malaria reduction (pre-elimination) in a previously high endemic region in sub-Saharan Africa. The data highlight constraints of optimistic prognostic modelling studies. New challenges, mainly with outdoor transmission, a large asymptomatic parasite reservoir and imported infections, require novel tools and reoriented strategies to prevent a rebound effect and achieve elimination.

Thermoregulation of Premature Infants during and after Skin-to-Skin Care

OBJECTIVE

Providing normothermia is an important issue in daily routine care of premature neonates. We recently found with infrared thermography (IRT) a drop in skin temperature of premature babies after they were positioned from skin-to-skin care (SSC) back into the incubator. Since this did not disappear within 10 min, we wanted to find out how long it takes until the baby has fully warmed up after SSC and if the IRT measurements correlate with conventional rectal temperature?

STUDY DESIGN

A prospective observational study was undertaken with 5 premature infants [3 male, median gestational age 28 weeks (25-29), median age at study 34 d (28-52), median birth weight 898 g (400-1095), median weight at study 1263 g (790-1465)], temperature was determined with IRT (leg, back, arm, head, upper abdomen; diameter 1 cm, scale 0.00°C), comparison with 2 conventional sensors and rectal temperature. Temperatures were recorded every 2 min and displayed for 4 time points, namely at the beginning and the end of skin-to-skin care (SSC1, SSC2), as well as at the beginning and the end of a subsequent 60 min incubator period (I).

RESULTS

A significant rise during SSC occurred while the cooling after SSC persisted during the complete incubator measurement time (I; p<0.05). Rectal temperature remained stable through the whole measuring period.

CONCLUSION

While SSC in our setting led to an increase in temperature, the lack of compensation of peripheral heat loss in the incubator after 60 min may express an inadequate peripheral regulation of body temperature. This should be taken into account before routine care after SSC.

Clinical "case series": a concept analysis

OBJECTIVES

To analyze the concept of "case series" in the medical literature compared with case reports.

METHODS

A PubMed search for articles published during 2009 which had "case series" in their title was performed. A total number of 621 articles were retrieved. 586 papers were included in the analysis and 35 were excluded (18 were commentary letters, 5 were not in English, and twelve could not be retrieved by our Library). The number of patients and category of these articles were analyzed.

RESULTS

The median (range) of the number of cases of articles having "case series" in their title was 7 (1-6432) cases. 186/ 586 articles had less than 5 cases (31.7%, 95% CI (28.3-35.1%)). The median (range) of the number of cases of articles having "case report" as their publication type was 4 (1-178) cases. Out of the 219 articles categorized as case reports 114 (52.1%, 95% CI (45.6-58.6%)) had less than five cases.

CONCLUSIONS

The concept of "case series" is not well defined in the literature and does not reflect a specific research design. We suggest that a case series should have more than four patients while four paitents or less should be reported individually as case reports.

Differentiation and tolerance of CD4+ T lymphocytes

The development of effector and memory populations of T lymphocytes is determined by antigen-induced growth and differentiation of naive T cells, and it is regulated by antigen-induced functional tolerance and cell death. CD4+ helper T lymphocytes that vary in their profiles of cytokine production and in effector functions also show distinct responses to antigens and co-stimulatory signals, and they differ in their sensitivity to tolerance induction. Thus, stimuli that trigger T cell growth and differentiation, as well as mechanisms that inhibit T cell expansion, determine both the magnitude and the nature of T cell-dependent immune responses to protein antigens.

CTLA-4 regulates induction of anergy in vivo

The requirement for CTLA-4 during the induction of peripheral T cell tolerance in vivo was investigated using naive TCR transgenic T cells lacking CTLA-4. CTLA-4(-/-) T cells are resistant to tolerance induction, as demonstrated by their proliferative responses, IL-2 production, and progression into the cell cycle. Following exposure to a tolerogenic stimulus in vivo and restimulation in vitro, wild-type T cells are blocked at the late G1 to S restriction point of the cell cycle. In contrast, CTLA-4(-/-) T cells enter into the S phase of the cell cycle, as shown by downregulation of p27(kip1), elevated cdk2 kinase activity, and Rb hyperphosphorylation. Thus, CTLA-4 has an essential role in determining the outcome of T cell encounter with a tolerogenic stimulus.

Akt provides the CD28 costimulatory signal for up-regulation of IL-2 and IFN-gamma but not TH2 cytokines

A region of the interleukin-2 (IL-2) promoter known as the RE/AP element is activated in concert by signals that originate from the T cell antigen receptor and the CD28 coreceptor. We show here that the serine-threonine kinase Akt can provide a costimulatory signal for RE/AP activation that is indistinguishable from the signal provided by CD28. This includes the ability of Akt, like antibodies to CD28, to synergize with protein kinase C theta (PKC-theta) in the induction of RE/AP. Retrovirus-mediated expression of activated Akt in primary T cells from CD28-deficient mice is capable of selectively restoring production of IL-2 and interferon gamma, but not IL-4 or IL-5. Our results provide evidence that CD28 costimulation of different cytokines is mediated by discrete signaling pathways, one of which includes Akt.

The roles of CD28 and CD40 ligand in T cell activation and tolerance

Costimulation of T cell activation involves both the B7:CD28 as well as the CD40 ligand (CD40L):CD40 pathway. To determine the importance of these pathways to in vitro and in vivo T cell activation, a direct comparison was made of the responses of TCR transgenic T cells lacking either CD28 or CD40L. In vitro, CD28-/- T cells showed a greater reduction in proliferative responses to Ag than did CD40L-/- T cells. The absence of CD28 resulted in defective Th2 responses, whereas CD40L-/- T cells were defective in Th1 development. In vivo, CD28-/- T cells failed to expand upon immunization, whereas CD40L-/- T cells could not sustain a response. These results suggest that CD28 is critical for initiating T cell responses, whereas CD40L is required for sustained Th1 responses. The different functional roles of these costimulatory pathways may explain why blocking B7:CD28 and CD40L:CD40 interactions has an additive effect in inhibiting T cell responses.

Combined effect of exposure to lead and chlordane on the testicular tissues of swiss mice

Chlordane, is one of many environmental pollutants that reach human or animal body through food and water consumption, its presence may frequently be associated with other metals such as lead. In this work, the toxicity of chlordane and lead to reproductive tissues in Swiss mice was studied. Oral daily doses of 75 and 275 mg/kg b.w. were administered to male mice, for 35 days, animals were sacrificed at the 2nd, 3rd, 4th, and 5th week. The results show that chloradane had a damaging effect on testicular tissues, the effect was obvious through reduction in diameter of the seminiferous tubules, number of spermatogonia, and primary and secondary spermatocytes and spermatids. The effect was remarkably increased by the presence of lead, which needs more investigation in order to determine whether it's a synergistic or additive effect.

Functional responses and costimulator dependence of memory CD4+ T cells

To examine the functional characteristics of memory CD4+ T cells, we used an adoptive transfer system to generate a stable population of Ag-specific memory cells in vivo and compared their responses to Ag with those of a similar population of Ag-specific naive cells. Memory cells localized to the spleen and lymph nodes of mice and exhibited extremely rapid recall responses to Ag in vivo, leaving the spleen within 3-5 days of Ag encounter. Unlike their naive counterparts, memory cells produced effector cytokines (IFN-gamma, IL-4, IL-5) within 12-24 h of Ag exposure and did not require multiple cycles of cell division to do so. Memory cells proliferated at lower Ag concentrations than did naive cells, were less dependent on costimulation by B7 molecules, and independent of costimulation by CD40. Furthermore, effector cytokine production by memory cells also occurred in the absence of either B7 or CD40 costimulation. Lastly, memory cells were resistant to tolerance induction. Together, these findings suggest that the threshold for activation of memory CD4+ cells is lower than that of naive cells. This would permit memory cells to rapidly express their effector functions in vivo earlier in the course of a secondary immune response, when the levels of Ag and the availability of costimulation may be relatively low.

Autoimmunity as a consequence of retrovirus-mediated expression of C-FLIP in lymphocytes

The induction of apoptosis by death receptors serves to regulate immune responses by eliminating unwanted and harmful cells. Mature lymphocytes express FLICE inhibitory proteins (FLIPs) that block death receptor-induced cell death. Here, we show that both B and T cells downregulate c-FLIP upon activation in vitro. Retrovirus-mediated expression of c-FLIP blocks Fas-induced apoptosis of activated lymphocytes but does not affect cell death resulting from cytokine withdrawal. In vivo, c-FLIP expression results in defective superantigen-mediated elimination of T cells, the accumulation of activated B cells, the production of autoantibodies, and the development of autoimmune disease. No effect was seen on negative selection of thyomocytes. These results suggest that activation-dependent downregulation of c-FLIP renders mature lymphocytes sensitive to death receptor-mediated apoptosis and is required to maintain self-tolerance.

The role of CTLA-4 in regulating Th2 differentiation

To examine the role of CTLA-4 in Th cell differentiation, we used two newly generated CTLA-4-deficient (CTLA-4-/-) mouse strains: DO11. 10 CTLA-4-/- mice carrying a class II restricted transgenic TCR specific for OVA, and mice lacking CTLA-4, B7.1 and B7.2 (CTLA-4-/- B7.1/B7.2-/- ). When purified naive CD4+ DO11.10 T cells from CTLA-4-/- and wild-type mice were primed and restimulated in vitro with peptide Ag, CTLA-4-/- DO11.10 T cells developed into Th2 cells, whereas wild-type DO11.10 T cells developed into Th1 cells. Similarly, when CTLA-4-/- CD4+ T cells from mice lacking CTLA-4, B7. 1, and B7.2 were stimulated in vitro with anti-CD3 Ab and wild-type APC, these CTLA-4-/- CD4+ T cells produced IL-4 even during the primary stimulation, whereas CD4+ cells from B7.1/B7.2-/- mice did not produce IL-4. Upon secondary stimulation, CD4+ T cells from CTLA-4-/- B7.1/B7.2-/- mice secreted high levels of IL-4, whereas CD4+ T cells from B7.1/B7.2-/- mice produced IFN-gamma. In contrast to the effects on CD4+ Th differentiation, the absence of CTLA-4 resulted in only a modest effect on T cell proliferation, and increased proliferation of CTLA-4-/- CD4+ T cells was seen only during secondary stimulation in vitro. Administration of a stimulatory anti-CD28 Ab in vivo induced IL-4 production in CTLA-4-/- B7.1/B7.2-/- but not wild-type mice. These studies demonstrate that CTLA-4 is a critical and potent inhibitor of Th2 differentiation. Thus, the B7-CD28/CTLA-4 pathway plays a critical role in regulating Th2 differentiation in two ways: CD28 promotes Th2 differentiation while CTLA-4 limits Th2 differentiation.

Uncoupling IL-2 signals that regulate T cell proliferation, survival, and Fas-mediated activation-induced cell death

IL-2 is an important growth and survival factor for T lymphocytes but also sensitizes these cells to Fas-mediated activation-induced cell death (AICD). The molecular basis of these different effects of IL-2 was studied by introducing wild-type and mutant forms of the IL-2 receptor beta (IL-2Rbeta) chain that lacked specific signaling capacities into receptor-deficient T cells by retroviral gene transfer. Activation of Stat5 by IL-2 was found to be involved in T cell proliferation and promoted Fas ligand (FasL) expression and AICD. T cell survival was dependent on a receptor region that activated Akt and the expression of Bcl-2. Thus, distinct IL-2Rbeta chain signaling modules regulate T cell fate by stimulating growth and survival or by promoting apoptosis.

Genetic models of abnormal apoptosis in lymphocytes

Apoptosis is a critical mechanism for regulating cell numbers during development, normal responses to hormones and other stimuli, and immune and inflammatory reactions. Recent advances in defining the biochemical mechanisms of cell death, and the development of animal models with isolated defects in cell death pathways, have led to an increasing appreciation of the pathophysiologic importance of lymphocyte apoptosis. In this article, we review our current understanding of the pathways and roles of apoptosis in lymphocytes, with an emphasis on transgenic and knockout models. We also summarize the relevance of these animal models to human diseases.

Functional characteristics and survival requirements of memory CD4+ T lymphocytes in vivo

The phenotypic and functional characteristics of Ag-specific memory CD4+ lymphocytes are poorly defined. To examine the properties and cytokine responsiveness of these cells, we have developed an adoptive transfer system using in vitro-activated T cells expressing the DO.11 transgenic TCR specific for OVA(323-339)+ I-Ad. In vitro-activated DO.11 CD4+ cells exhibit comparable survival patterns at 1, 6, and 10 wk after adoptive transfer, indicating that a stable population of memory cells has been generated. In the absence of Ag, previously activated T cells survive longer than their naive counterparts in vivo, rapidly revert to a partially naive phenotype, and maintain their effector cytokine profile. The DO.11 CD4+ memory cells are capable of proliferating in response to IL-2 and IL-4, while naive DO.11 CD4+ cells exhibit no such proliferative responses.

Mechanisms of peripheral T cell tolerance

Peripheral tolerance to self proteins is induced because these antigens are presented to T lymphocytes under conditions that do not allow effective immune responses to develop, or because the responses of the specific T cells are tightly regulated. The two principal mechanisms of peripheral tolerance are activation-induced cell death (AICD) and anergy. In CD4+ T lymphocytes, AICD is induced by repeated stimulation, with high levels of interleukin (IL)-2 production. Under these conditions, the T cells co-express Fas (CD95) and Fas ligand (FasL), and engagement of Fas triggers apoptotic death of the T cells. Mice with defects in Fas, FasL, IL-2R alpha or beta chain exhibit defects in AICD and develop autoimmune disease. The induction of T cell anergy is dependent on the recognition of B7 co-stimulators by the inhibitory T cell counter-receptor, CTLA-4. Failure of anergy is the likely basis for the fatal autoimmune disease of CTLA-4 knockout mice. The single-gene defects that result in autoimmunity are all defects in lymphocyte regulation, indicating that tolerance is often maintained by the control of lymphocyte responses to self antigen. The existence of distinct pathways of T cell tolerance suggest that different types of antigens induce tolerance by distinct mechanisms.

Role of cytokines in autoimmunity

Activation-induced apoptosis of T lymphocytes is an important mechanism for maintaining self-tolerance. The sensitivity of T cells to apoptosis by the Fas pathway is regulated by the exposure of these cells to different cytokines. IL-2 is a survival and growth factor for T cells, as well as a necessary potentiator of Fas-mediated cell death. The role of this cytokine in triggering death pathways is the likely explanation for the autoimmune disease that develops as a result of targeted disruption of the IL-2 or IL-2 receptor alpha or beta chain gene.

Role of mu heavy chain in B cell development. I. Blocked B cell maturation but complete allelic exclusion in the absence of Ig alpha/beta

There is good evidence for a signaling role played by Ig heavy chain in the developmental transition through the pre-B cell stage. We have previously described signal-capable or signal-incapable mutants of mu heavy chain in which a signaling defect is caused by failure to associate with the Ig alpha/beta heterodimer. To further characterize the role of Ig heavy chain-mediated signaling in vivo, as well as in B cell development and allelic exclusion, we have created transgenic mice in which the B cells express these signal-capable and signal-incapable mutant mu chains. Failure of mu to signal via Ig alpha/beta results in a block in B cell development in mice expressing the signal-incapable mu. A small number of B cells in these animals do escape the developmental block and are expressed in the spleen and the periphery as B220+ transgenic IgM+ cells. These cells respond to LPS by proliferating but show no response to T-independent-specific Ag. In contrast, B cells expressing the signal-capable B cell receptor show a strong signaling response to Ag-specific stimulus. There is no Ig alpha seen in association with signal-deficient IgM. Thus, the B cell receptor complex is not assembled, and no signal can be delivered. Despite the block in developmental signaling, allelic exclusion is complete. There is no detectable coexpression of transgenic IgM and endogenous murine IgM, nor is there rearrangement of the endogenous heavy chain genes. This suggests that differing signaling mechanisms are responsible for the developmental transition and allelic exclusion and thus allows for separate examination of these signaling mechanisms.

Helper T cell subsets: heterogeneity, functions and development

The discovery that the nature of cytokine production by CD4+ T lymphocytes could drastically alter an immune response led to the categorization of distinct helper T cell subsets, most notably Th1 and Th2. Recent evidence suggests that such helper responses are actually quite heterogeneous and ultimately, the course of an immune response depends upon the predominance of particular cytokines. While the factors leading to the production of individual cytokines are not completely defined, it is clear that the nature and dose of antigen, location of antigen challenge, and genetic composition of the individual all play a role in the process. Elucidating the cellular and molecular pathways responsible for helper T cell differentiation will ultimately permit the manipulation of immune responses to pathogens, as well as the development of novel vaccine strategies.

Biochemical mechanisms of IL-2-regulated Fas-mediated T cell apoptosis

Activation-induced cell death (AICD) of lymphocytes is an important mechanism of self-tolerance. In CD4+ T cells, AICD is mediated by the Fas pathway and is enhanced by IL-2. To define the mechanisms of this pro-apoptotic action of IL-2, we analyzed CD4+ T cells from wild-type and IL-2-/- mice expressing a transgenic T cell receptor. T cells become sensitive to AICD after activation by antigen and IL-2. IL-2 increases transcription and surface expression of Fas ligand (FasL) and suppresses transcription and expression of FLIP, the inhibitor of apoptosis. The ability of IL-2 to enhance expression of a pro-apoptotic molecule, FasL, and to suppress an inhibitor of Fas signaling, FLIP, likely accounts for the role of this cytokine in potentiating T cell apoptosis.

Homeostasis and self-tolerance in the immune system: turning lymphocytes off

The immune system responds in a regulated fashion to microbes and eliminates them, but it does not respond to self-antigens. Several regulatory mechanisms function to terminate responses to foreign antigens, returning the immune system to a basal state after the antigen has been cleared, and to maintain unresponsiveness, or tolerance, to self-antigens. Here, recent advances in understanding of the molecular bases and physiologic roles of the mechanisms of immune homeostasis are examined.

Functional roles of Fas and Bcl-2-regulated apoptosis of T lymphocytes

Apoptotic cell death is an important mechanism for maintaining homeostasis in the immune system and for regulating the fates of lymphocytes following encounters with self and foreign Ags. To study the physiologic roles of the proapoptotic Fas pathway and the antiapoptotic protein, Bcl-2, in T cell maturation and homeostasis, a TCR transgene has been bred into mice lacking functional Fas and mice that express Bcl-2 constitutively. In vitro, Fas-deficient T cells are resistant to activation-induced cell death, whereas Bcl-2-overexpressing T cells are resistant to death induced by withdrawal of growth factors. In vivo, Bcl-2-overexpressing mice accumulate T cells in the thymus and peripheral lymphoid tissues in the absence of Ag, but these cells are deleted normally after Ag administration. In contrast, Fas-deficient mature T cells are present in normal numbers in the absence of Ag, but are resistant to Ag-induced deletion. Both Fas-deficient and Bcl-2 overexpressing thymocytes are deleted when exposed to transgene-encoded circulating self Ag, indicating that the pathways of apoptosis controlled by these proteins are not critical for negative selection of developing thymocytes. Moreover, deficiency of Fas, but not Bcl-2 overexpression, results in the accumulation of autoreactive T cells in peripheral lymphoid tissues. These results demonstrate that Fas and Bcl-2 regulate different pathways of apoptosis that may serve distinct functions in lymphocyte homeostasis and in the maintenance of T cell tolerance.

The Fas/Fas ligand pathway and Bcl-2 regulate T cell responses to model self and foreign antigens

We have examined the role of Fas and Bcl-2 in T cell survival and responses to antigen in vivo using T cells that express a transgenic antigen receptor specific for hen egg lysozyme (HEL) and that either lack functional Fas or Fas ligand (FasL) or overexpress Bcl-2 as a transgene. HEL-specific, Bcl-2-transgenic T cells showed prolonged responses to immunization with cognate peptide but were eliminated rapidly when exposed to HEL expressed systemically as a self antigen. In contrast, Fas- and FasL-defective T cells did not display exaggerated responses to immunization with HEL peptide, but did show increased expansion and survival in response to systemic self antigen and were able to activate anti-HEL (self) antibody-forming cells. Thus, Bcl-2 and Fas play different roles in the regulation of T cell responses to antigen in vivo and in self tolerance.

Functional consequences of dysregulated B7-1 (CD80) and B7-2 (CD86) expression in B or T lymphocytes of transgenic mice

T cell activation and tolerance are regulated by interactions between CD28 or CTLA-4 on T cells and B7 costimulatory molecules on APCs. We have generated transgenic mouse strains that constitutively express B7-1 (CD80) at high levels on B cells or T cells or express B7-2 (CD86) on T lymphocytes to examine the consequences of dysregulated B7 expression on T cell responses. The transgene-derived B7 molecules are functional, because B7-1 transgenic B cells are more efficient APCs than are wild-type B cells, and the activation of B7 transgenic T cells is less dependent on exogenous costimulation than that of wild-type T cells. In vivo, constitutive expression of B7 molecules leads to the elimination of immature B cells. The expression of B7 molecules on thymocytes results in the down-regulation of CD28 expression. However, B7 transgenic mice have normal numbers of mature lymphocytes and mount normal T cell responses following immunization with protein Ag. Neither anergy induction nor superantigen-mediated deletion of T cells is altered by the dysregulated expression of B7-1 or B7-2 on B or T lymphocytes in these transgenic strains. Therefore, functionally significant levels of B7 expressed constitutively on mature lymphocytes are not, by themselves, sufficient to abrogate T cell tolerance or induce autoimmune disease.

Functional consequences of dysregulated B7-1 (CD80) and B7-2 (CD86) expression in B or T lymphocytes of transgenic mice

T cell activation and tolerance are regulated by interactions between CD28 or CTLA-4 on T cells and B7 costimulatory molecules on APCs. We have generated transgenic mouse strains that constitutively express B7-1 (CD80) at high levels on B cells or T cells or express B7-2 (CD86) on T lymphocytes to examine the consequences of dysregulated B7 expression on T cell responses. The transgene-derived B7 molecules are functional, because B7-1 transgenic B cells are more efficient APCs than are wild-type B cells, and the activation of B7 transgenic T cells is less dependent on exogenous costimulation than that of wild-type T cells. In vivo, constitutive expression of B7 molecules leads to the elimination of immature B cells. The expression of B7 molecules on thymocytes results in the down-regulation of CD28 expression. However, B7 transgenic mice have normal numbers of mature lymphocytes and mount normal T cell responses following immunization with protein Ag. Neither anergy induction nor superantigen-mediated deletion of T cells is altered by the dysregulated expression of B7-1 or B7-2 on B or T lymphocytes in these transgenic strains. Therefore, functionally significant levels of B7 expressed constitutively on mature lymphocytes are not, by themselves, sufficient to abrogate T cell tolerance or induce autoimmune disease.

Role of interleukin 12 and costimulators in T cell anergy in vivo

The induction of T cell anergy in vivo is thought to result from antigen recognition in the absence of co-stimulation and inflammation, and is associated with a block in T cell proliferation and Th1 differentiation. Here we have examined the role of interleukin (IL)-12, a potent inducer of Th1 responses, in regulating this process. T cell tolerance was induced by the administration of protein antigen without adjuvant in normal mice, and in recipients of adoptively transferred T cells from T cell receptor transgenic mice. The administration of IL-12 at the time of tolerance induction stimulates Th1 differentiation, but does not promote antigen-specific T cell proliferation. Conversely, inhibiting CTLA-4 engagement during anergy induction reverses the block in T cell proliferation, but does not promote full Th1 differentiation. T cells exposed to tolerogenic antigen in the presence of both IL-12 and anti-CTLA-4 antibody are not anergized, and behave identically to T cells which have encountered immunogenic antigen. These results suggest that two processes contribute to the induction of anergy in vivo; CTLA-4 engagement, which leads to a block in the ability of T cells to proliferate to antigen, and the absence of a prototypic inflammatory cytokine, IL-12, which prevents the differentiation of T cells into Th1 effector cells. The combination of IL-12 and anti-CTLA-4 antibody is sufficient to convert a normally tolerogenic stimulus to an immunogenic one.

Functional responses and apoptosis of CD25 (IL-2R alpha)-deficient T cells expressing a transgenic antigen receptor

IL-2 was initially defined as a T lymphocyte growth factor, but recent studies have provided evidence that it may also play a role in regulating T cell differentiation, apoptosis, and tolerance. To examine the contribution of IL-2 to these processes, we have bred a class II-restricted TCR transgene into mice deficient in the alpha-chain of the IL-2R, CD25. We show that in response to Ag, T cells from these mice are unable to use IL-2 and, as a result, are less efficient at traversing the cell cycle, and proliferate less than wild-type cells. Furthermore, CD25 -/- T cells exhibit reduced survival in vitro, even in the presence of costimulatory signals. IL-4 and IL-15, a cytokine related to IL-2, enhance the survival and Ag-induced proliferation of CD25 -/- T cells. Activated CD25 -/- T cells are resistant to Fas-mediated activation-induced cell death (AICD), and this defect cannot be corrected by other cytokines. Therefore, IL-2 plays a unique role in regulating AICD, but has redundant roles in T cell survival and proliferation in vitro. The failure of AICD observed with CD25 -/- T cells may explain the unexpected observation that deficiency of IL-2 or of the alpha- or beta-chain of the IL-2R results not in immunodeficiency, but in autoimmune disease.

Functional responses and apoptosis of CD25 (IL-2R alpha)-deficient T cells expressing a transgenic antigen receptor

IL-2 was initially defined as a T lymphocyte growth factor, but recent studies have provided evidence that it may also play a role in regulating T cell differentiation, apoptosis, and tolerance. To examine the contribution of IL-2 to these processes, we have bred a class II-restricted TCR transgene into mice deficient in the alpha-chain of the IL-2R, CD25. We show that in response to Ag, T cells from these mice are unable to use IL-2 and, as a result, are less efficient at traversing the cell cycle, and proliferate less than wild-type cells. Furthermore, CD25 -/- T cells exhibit reduced survival in vitro, even in the presence of costimulatory signals. IL-4 and IL-15, a cytokine related to IL-2, enhance the survival and Ag-induced proliferation of CD25 -/- T cells. Activated CD25 -/- T cells are resistant to Fas-mediated activation-induced cell death (AICD), and this defect cannot be corrected by other cytokines. Therefore, IL-2 plays a unique role in regulating AICD, but has redundant roles in T cell survival and proliferation in vitro. The failure of AICD observed with CD25 -/- T cells may explain the unexpected observation that deficiency of IL-2 or of the alpha- or beta-chain of the IL-2R results not in immunodeficiency, but in autoimmune disease.

T-cell subsets: recruiting the right kind of help

The qualitative features of immune responses are influenced by the polarization of helper T cells towards two distinct phenotypes, Th1 and Th2. Recent evidence suggests that these helper cell subsets may be differentially recruited to the sites of different types of inflammatory reaction.

Induction of peripheral T cell tolerance in vivo requires CTLA-4 engagement

Studies of T cell anergy in vitro have led to the widely accepted view that anergy is induced by T cell antigen recognition without costimulation. We show that the induction of T cell anergy in vivo is due to an abortive T cell response that requires recognition of B7 molecules, since blocking B7 maintains T cells in an unactivated but functionally competent state. Furthermore, the induction of anergy is prevented by blocking CTLA-4, the inhibitory T cell receptor for B7 molecules. Thus, in vivo T cell anergy may be induced not because of a lack of costimulation, but as a result of specific recognition of B7 molecules by CTLA-4. In contrast, blocking CD28 on T cells prevents priming but not the induction of tolerance. Therefore, the outcome of antigen recognition by T cells is determined by the interaction of CD28 or CTLA-4 on the T cells with B7 molecules.

Role of costimulators in T cell differentiation: studies using antigen-presenting cells lacking expression of CD80 or CD86

For T cells to be optimally activated, recognition of Ag/MHC complexes by the TCR must be accompanied by a second, costimulatory signal that can be provided efficiently by the related costimulatory molecules CD80 (B7-1) and CD86 (B7-2). Recently, CD80 and CD86 have been implicated as differential determinants of Th1- vs Th2-type cytokine profiles. However, this remains a controversial issue since conflicting results have been obtained in different experimental models both in vivo and in vitro. To investigate the role of CD80 and CD86 in Th subset differentiation, we have examined the cytokine profiles induced in TCR transgenic T cells stimulated by peptide in association with splenic APCs obtained from knockout mice that selectively lack expression of either the CD80 or the CD86 molecule. Our data suggest that CD86, and to a lesser extent CD80, can make significant contributions to the production of both IL-4 and IFN-gamma. However, neither molecule plays an obligatory role in priming for the production of either effector cytokine. Furthermore, CD80 and CD86 contribute to the magnitude of T cell activation, but do not appear to selectively regulate Th1 vs Th2 differentiation.

Role of costimulators in T cell differentiation: studies using antigen-presenting cells lacking expression of CD80 or CD86

For T cells to be optimally activated, recognition of Ag/MHC complexes by the TCR must be accompanied by a second, costimulatory signal that can be provided efficiently by the related costimulatory molecules CD80 (B7-1) and CD86 (B7-2). Recently, CD80 and CD86 have been implicated as differential determinants of Th1- vs Th2-type cytokine profiles. However, this remains a controversial issue since conflicting results have been obtained in different experimental models both in vivo and in vitro. To investigate the role of CD80 and CD86 in Th subset differentiation, we have examined the cytokine profiles induced in TCR transgenic T cells stimulated by peptide in association with splenic APCs obtained from knockout mice that selectively lack expression of either the CD80 or the CD86 molecule. Our data suggest that CD86, and to a lesser extent CD80, can make significant contributions to the production of both IL-4 and IFN-gamma. However, neither molecule plays an obligatory role in priming for the production of either effector cytokine. Furthermore, CD80 and CD86 contribute to the magnitude of T cell activation, but do not appear to selectively regulate Th1 vs Th2 differentiation.

Bicarbonate-sensitive cysteine induced elevation of extracellular aspartate and glutamate in rat hippocampus in vitro

The effect of different concentrations of cysteine (0.125, 0.25, 0.5 and 1 mM) on the net efflux of endogenous amino acids was studied by the incubation of rat hippocampal slices. Addition of cysteine (1 mM) in bicarbonate containing low K+ medium (5 min) selectively increased the basal net efflux of glutamate and aspartate by 370% and 396%, respectively. High K+ media (50 mM) containing cysteine (1 mM) evoked the net efflux of glutamate and aspartate by 1454% and 1019%, respectively. The corresponding effects in control slices without cysteine were 669% and 404%, respectively. No changes were observed on the concentrations of GABA, glutamine and taurine. The cysteine oxidation products, cysteine sulfinate (0.5 microM) and cystine (0.25 mM) were without effects. The effect of cysteine (0.5 mM) was dramatically reduced in media with no added bicarbonate/CO2. Thus, cysteine in a bicarbonate-sensitive manner selectively increases the extracellular concentration of excitotoxic amino acids in adult rat brain in vitro, possibly by interfering with the carrier-mediated glutamate uptake/release.

Functional diversity of helper T lymphocytes

The existence of subsets of CD4+ helper T lymphocytes that differ in their cytokine secretion patterns and effector functions provides a framework for understanding the heterogeneity of normal and pathological immune responses. Defining the cellular and molecular mechanisms of helper-T-cell differentiation should lead to rational strategies for manipulating immune responses for prophylaxis and therapy.

Cytokine transcriptional events during helper T cell subset differentiation

The molecular basis for changes in cytokine expression during T helper (Th) cell subset differentiation is not well understood. We have characterized transcriptional events related to cytokine gene expression in populations of naive T cell receptor-transgenic T cells as they are driven in vitro toward Th1 or Th2 phenotypes by interleukin (IL)-12 or IL-4 treatment, respectively. Quantitative reverse transcriptase-polymerase chain reaction analysis of cytokine transcripts indicates that interferon (IFN) gamma, IL-4, and IL-2 mRNA are expressed with distinct kinetics after naive T cells are stimulated with antigen and either IL-4 or IL-12. IFN-gamma mRNA appears as early as 6 h in IL-12-treated cultures, IL-4 appears only after 48 h in IL-4-treated cultures, and IL-2 is equivalently expressed in both types of cultures. Analyses were performed to determine if there were any differences in activation of IL-2 or IL-4 transcription factors that accompanied Th1 versus Th2 differentiation. These studies demonstrated that signal transducer and activator of transcription 6 (STAT6) binds to a sequence in the IL-4 promoter and that this STAT6-binding site can support IL-4-dependent transcription of a linked heterologous promoter. Prolonged activation of STAT6 is characteristic of populations undergoing Th2 differentiation. Furthermore, STAT6 is activated in an autocrine manner when differentiated Th2 populations are stimulated by antigen receptor ligation. Th1 populations derived from IL-12 plus antigen treatment of naive T cells remain responsive to IL-4 as indicated by induction of STAT6 and IL-4 mRNA. These data indicate that Th1 and Th2 differentiation represents the combination of different, apparently independently regulated transcriptional events. Furthermore, among transcription factors that bind to the IL-4 or IL-2 promoters, STAT6 is the one whose activation distinguishes Th2 versus Th1 development.

Role of Fas-mediated cell death in the regulation of immune responses

Interaction of the cell-surface molecule Fas (CD95 APO-1) with its specific ligand triggers apoptotic death of T and B lymphocytes. This pathway is important for eliminating self-reactive lymphocytes and thus preventing autoimmunity. Fas is also involved in controlling injurious lymphocyte reactions in immunologically 'privileged' tissues, and may provide a strategy for reducing graft rejection.

The roles of costimulation and Fas in T cell apoptosis and peripheral tolerance

Using cells from TCR transgenic mice that do or do not express Fas, we show that there are two mechanistically distinct forms of apoptosis in CD4+ T cells. Naive T cells undergo apoptosis if cultured in the absence of antigen or costimulation. This form of programmed cell death (PCD) is not dependent on Fas, and is prevented by CD28-mediated signals, which lead to the secretion of growth factors and the expression of survival genes, such as bcl-xL. Recently activated T cells undergo apoptotic death upon repeated stimulation. This activation-induced cell death (AICD) is mediated by Fas, but is independent of costimulation and is not prevented by IL-2 or bcl-xL. Finally, we show that peripheral tolerance may be induced in vivo independent of Fas-mediated cell death.

T cell responses in calcineurin A alpha-deficient mice

We have created embryonic stem (ES) cells and mice lacking the predominant isoform (alpha) of the calcineurin A subunit (CNA alpha) to study the role of this serine/threonine phosphatase in the immune system. T and B cell maturation appeared to be normal in CNA alpha -/- mice. CNA alpha -/- T cells responded normally to mitogenic stimulation (i.e., PMA plus ionomycin, concanavalin A, and anti-CD3 epsilon antibody). However, CNA alpha -/- mice generated defective antigen-specific T cell responses in vivo. Mice produced from CNA alpha -/- ES cells injected into RAG-2-deficient blastocysts had a similar defective T cell response, indicating that CNA alpha is required for T cell function per se, rather than for an activity of other cell types involved in the immune response. CNA alpha -/- T cells remained sensitive to both cyclosporin A and FK506, suggesting that CNA beta or another CNA-like molecule can mediate the action of these immunosuppressive drugs. CNA alpha -/- mice provide an animal model for dissecting the physiologic functions of calcineurin as well as the effects of FK506 and CsA.

IL-12, as an adjuvant, promotes a T helper 1 cell, but does not suppress a T helper 2 cell recall response

IL-12 is a potent inducer of NK and cytolytic T cell activity, IFN-gamma production, and T cell proliferation, and is necessary for differentiation of naive T cells to the Th1 subset. We have previously shown that IL-12 promotes a primary Th1 response and suppresses a primary Th2 response in lymph nodes of mice primed with a model hapten-protein conjugate, 2,4,6-trinitrophenyl (TNP)-keyhole limpet hemocyanin (KLH). We have now extended these studies to determine the Th phenotype of the recall response following immunization with soluble Ag and IL-12. For these experiments, mice were primed with TNP-KLH with or without treatment with IL-12, allowed to progress beyond the primary immune response, and challenged by i.p. injection of TNP-KLH. The phenotype of the recall response was monitored by measuring ex vivo production of IFN-gamma and IL-4 in Ag-stimulated lymph node and spleen cell cultures. Titer and isotype of TNP-specific serum Abs were also evaluated. Mice primed with Ag+IL-12 developed a Th1 recall response, as detected by KLH-specific IFN-gamma production from cultured spleen cells and the presence of TNP-specific IgG2a Ab in serum. However, they also developed an Ag-specific Th2 recall response, as characterized by Ag-induced IL-4 production from spleen cells and the presence of high titers of anti-TNP IgG1 in the serum. Studies of the cytokine profile during the primary response revealed that IL-12 induced in spleen cells the capacity to express both IL-4 and IFN-gamma. CD4+ T cells are necessary for production of IL-4 in the spleens of IL-12-treated mice, and most likely account for the Th2 recall response detected in mice primed with Ag+IL-12. These results indicate that the Th1 phenotype induced by immunization with IL-12 and Ag is maintained so that a Th1 recall response is expressed upon subsequent challenge with Ag. However, immunization with IL-12 also supports the development of a Th2 recall response, indicating that the Th1-inducing effect of IL-12 in vivo is not accompanied by a long lasting suppression of Th2 development.

IL-12, as an adjuvant, promotes a T helper 1 cell, but does not suppress a T helper 2 cell recall response

IL-12 is a potent inducer of NK and cytolytic T cell activity, IFN-gamma production, and T cell proliferation, and is necessary for differentiation of naive T cells to the Th1 subset. We have previously shown that IL-12 promotes a primary Th1 response and suppresses a primary Th2 response in lymph nodes of mice primed with a model hapten-protein conjugate, 2,4,6-trinitrophenyl (TNP)-keyhole limpet hemocyanin (KLH). We have now extended these studies to determine the Th phenotype of the recall response following immunization with soluble Ag and IL-12. For these experiments, mice were primed with TNP-KLH with or without treatment with IL-12, allowed to progress beyond the primary immune response, and challenged by i.p. injection of TNP-KLH. The phenotype of the recall response was monitored by measuring ex vivo production of IFN-gamma and IL-4 in Ag-stimulated lymph node and spleen cell cultures. Titer and isotype of TNP-specific serum Abs were also evaluated. Mice primed with Ag+IL-12 developed a Th1 recall response, as detected by KLH-specific IFN-gamma production from cultured spleen cells and the presence of TNP-specific IgG2a Ab in serum. However, they also developed an Ag-specific Th2 recall response, as characterized by Ag-induced IL-4 production from spleen cells and the presence of high titers of anti-TNP IgG1 in the serum. Studies of the cytokine profile during the primary response revealed that IL-12 induced in spleen cells the capacity to express both IL-4 and IFN-gamma. CD4+ T cells are necessary for production of IL-4 in the spleens of IL-12-treated mice, and most likely account for the Th2 recall response detected in mice primed with Ag+IL-12. These results indicate that the Th1 phenotype induced by immunization with IL-12 and Ag is maintained so that a Th1 recall response is expressed upon subsequent challenge with Ag. However, immunization with IL-12 also supports the development of a Th2 recall response, indicating that the Th1-inducing effect of IL-12 in vivo is not accompanied by a long lasting suppression of Th2 development.

Anatomy of autoantibody production: dominant localization of antibody-producing cells to T cell zones in Fas-deficient mice

The goal of this study was to examine the in vivo site of autoantibody production in normal and autoimmune-prone mice. B cells were identified in tissue sections with IgM- and IgG2a-specific riboprobes that readily distinguished resting cells from antibody-forming cells (AFC). In normal mice, the few identifiable IgG2a-secreting cells were found in the red pulp. By contrast, in Ipr mice exceedingly high numbers of IgG2a and autoantibody-producing cells were found deep within the T cell-rich periarteriolar lymphoid sheaths (PALS). This unusual anatomic location of autoantibody-secreting B cells is unique to Fas dysregulated strains, since IgG2-producing cells in MRL/+ and (SWR x NZB)F1 mice were found predominantly in the red pulp or outer PALS, similar to normal mice. Furthermore, analysis of spleens from Ipr and non-Ipr anti-DNA immunoglobulin transgenic mice revealed dramatic accumulation of Tg+ cells in the inner PALS only in Ipr mice. These data suggest that in the absence of Fas, autoreactive B cells accumulate in T cell-rich zones, and this anatomic feature may contribute to autoantibody production.

Intracellular targeting of antigens internalized by membrane immunoglobulin in B lymphocytes

An important function of membrane immunoglobulin (mIg), the B cell antigen receptor, is to endocytose limiting quantities of antigen for efficient presentation to class II-restricted T cells. We have used a panel of mIg mutants to analyze the mechanism of mIg-mediated antigen presentation, and specifically to explore the ability of mIg to target internalized antigen to intracellular processing compartments. Transfected mIgs carrying substitutions for the transmembrane Tyr587 residue fail to efficiently present specifically bound antigen. However, these mutants internalize antigen normally, and their defect cannot be attributed to a lack of mIg-associated Ig alpha/Ig beta molecules. A novel functional assay for detecting antigenic peptides in subcellular fractions shows that wild-type mIg transfectants generate class II-peptide complexes intracellularly, whereas only free antigenic peptides are detectable in the mutant mIg transfectants. Furthermore, an antigen competition assay reveals that antigen internalized by the mutant mIgs fails to enter the intracellular processing compartment accessed by wild-type mIg. Therefore, mIg specifically targets bound and endocytosed antigen to the intracellular compartment where processed peptides associate with class II molecules, and the transmembrane Tyr587 residue plays an obligatory role in this process. Targeting of internalized antigen may be mediated by receptor-associated chaperones, and may be a general mechanism for optimizing the presentation of specifically bound and endocytosed antigens in b lymphocytes and other antigen-presenting cells.

Stability of Th1 and Th2 populations

Using an in vitro model for the development of IFN-gamma-producing (Th1) and IL-4-producing (Th2) cells from CD4+ T lymphocytes expressing a transgenic TCR, we show that IL-12 and IL-4 are the most potent stimuli for the differentiation of naive T cells to effector populations. When combinations of cytokines are present during T cell priming, the effect of IL-4 is dominant. Furthermore, differentiated Th1 cells can be converted into IL-4 producers by exposure to IL-4, but the Th2 phenotype is not reversible. The stability of Th2 populations may limit the ability to regulate Th2-dominant responses in pathologic situations.

Molecular mechanisms that control expression of the B lymphocyte antigen receptor complex

The B cell receptor for antigen (BCR) is a complex of membrane immunoglobulin (mIg) and at least two other proteins, Ig alpha (mb-1) and Ig beta (B29). This complex promotes surface expression of the BCR and acts to transduce an activation signal. We have used a system of mu heavy chain constructs transfected into murine B cell lines to probe structure-function relationships in the BCR complex. One mutant mu chain, in which two polar transmembrane residues (Tyr587, Ser588) are replaced with valine, fails to associate with Ig alpha and Ig beta and is incapable of transducing signals as a result of mIg cross-linking. This mutant is expressed on the surface at high levels when transfected into a plasmacytoma line that lacks Ig alpha, whereas wild-type mu is retained in this cell line in the endoplasmic reticulum. Pulse-chase and immunoprecipitation analyses indicate that the mutant is more rapidly released from calnexin than the wild-type mu. Further, transfection of Ig alpha into this Ig alpha-negative cell line allows release of the mu chain from calnexin and surface expression of the BCR. These results identify the transmembrane residues of mu heavy chain that control binding to calnexin and Ig alpha, and suggest that calnexin-dependent intracellular retention is an important control mechanism for expression of the BCR complex.