Liver transplantation in ornithine transcarbamylase deficiency: A retrospective multicentre cohort study

Ornithine transcarbamylase deficiency (OTCD) is an X-linked defect of ureagenesis and the most common urea cycle disorder. Patients present with hyperammonemia causing neurological symptoms, which can lead to coma and death. Liver transplantation (LT) is the only curative therapy, but has several limitations including organ shortage, significant morbidity and requirement of lifelong immunosuppression. This study aims to identify the characteristics and outcomes of patients who underwent LT for OTCD. We conducted a retrospective study for OTCD patients from 5 UK centres receiving LT in 3 transplantation centres between 2010 and 2022. Patients' demographics, family history, initial presentation, age at LT, graft type and pre- and post-LT clinical, metabolic, and neurocognitive profile were collected from medical records. A total of 20 OTCD patients (11 males, 9 females) were enrolled in this study. 6/20 had neonatal and 14/20 late-onset presentation. 2/20 patients had positive family history for OTCD and one of them was diagnosed antenatally and received prospective treatment. All patients were managed with standard of care based on protein-restricted diet, ammonia scavengers and supplementation with arginine and/or citrulline before LT. 15/20 patients had neurodevelopmental problems before LT. The indication for LT was presence (or family history) of recurrent metabolic decompensations occurring despite standard medical therapy leading to neurodisability and quality of life impairment. Median age at LT was 10.5 months (6-24) and 66 months (35-156) in neonatal and late onset patients, respectively. 15/20 patients had deceased donor LT (DDLT) and 5/20 had living related donor LT (LDLT). Overall survival was 95% with one patient dying 6 h after LT. 13/20 had complications after LT and 2/20 patients required re-transplantation. All patients discontinued dietary restriction and ammonia scavengers after LT and remained metabolically stable. Patients who had neurodevelopmental problems before LT persisted to have difficulties after LT. 1/5 patients who was reported to have normal neurodevelopment before LT developed behavioural problems after LT, while the remaining 4 maintained their abilities without any reported issues. LT was found to be effective in correcting the metabolic defect, eliminates the risk of hyperammonemia and prolongs patients' survival.

Three-Country Snapshot of Ornithine Transcarbamylase Deficiency

X-linked ornithine transcarbamylase deficiency (OTCD) is the most common urea cycle defect. The disease severity ranges from asymptomatic carrier state to severe neonatal presentation with hyperammonaemic encephalopathy. We audited the diagnosis and management of OTCD, using an online 12-question-survey that was sent to 75 metabolic centres in Turkey, France and the UK. Thirty-nine centres responded and 495 patients were reported in total. A total of 208 French patients were reported, including 71 (34%) males, 86 (41%) symptomatic and 51 (25%) asymptomatic females. Eighty-five Turkish patients included 32 (38%) males, 39 (46%) symptomatic and 14 (16%) asymptomatic females. Out of the 202 UK patients, 66 (33%) were male, 83 (41%) asymptomatic and 53 (26%) symptomatic females. A total of 19%, 12% and 7% of the patients presented with a neonatal-onset phenotype in France, Turkey and the UK, respectively. Vomiting, altered mental status and encephalopathy were the most common initial symptoms in all three countries. While 69% in France and 79% in Turkey were receiving protein restriction, 42% were on a protein-restricted diet in the UK. A total of 76%, 47% and 33% of patients were treated with ammonia scavengers in Turkey, France and the UK, respectively. The findings of our audit emphasize the differences and similarities in manifestations and management practices in three countries.

P-122 Optimising IUI; a systematic review and network meta-analysis

Study question

What is the effectiveness of add-on interventions to the standard intra-uterine insemination (IUI) protocol in improving reproductive outcomes?

Summary answer

Amongst the add-ons studied only luteal phase support and endometrial scratch were found beneficial for (LBR)/ongoing pregnancy rate (OPR) in IUI cycles.

What is known already

IUI is a fertility treatment offered to couples and single women for varied indications worldwide. Although cheaper and less invasive than in-vitro fertilization (IVF), it has received less attention and success rates remain low. Various add-ons have been introduced to boost IUI outcomes. However, their use remains largely empirical and is not standardized. Exploring the effectiveness of different protocol add-ons in comparisons with each other and with the standard protocol, could help develop evidence-based recommendations and optimize IUI treatment.

Study design, size, duration

We conducted a systematic review and meta-analysis aiming to assess the value of various IUI add-ons on clinical outcomes (PROSPERO registration number CRD42022300857). A computerized literature search was performed using EMBASE, MEDLINE, CINAHL and the Cochrane Central from database inception to October 2021. Two authors independently assessed the studies for quality and risk of bias. Studied add-ons included use of hydrotubation, endometrial scratch, trigger, double insemination, ultrasound guidance, bed rest, tocolysis and luteal phase support.

Participants/materials, setting, methods

Randomized controlled trials (RCTs) were included, reporting on one or more cycles of IUI with any protocol and indication using partner’s or donor sperm. We summarized the LBR or OPR when LBR was not available. We calculated odds ratios with 95% confidence intervals (CI) using random effects meta-analysis after transforming data using Freeman-Tukey double arcsine transformation. Heterogeneity was reported as I2 and Tau2 estimates.

Main results and the role of chance

Sixty one RCTs were included in the analysis. Amongst the add-ons studied, luteal phase support and endometrial scratch were found to increase chance of LBR/OPR by 1.48 times (CI 1.1243-1.9402, I2= 13.3%, Tau2= 0,0156 p = 0.330) and 1.58 times (CI 1.0992-2.2685, I2= 0.0%,Tau2=0,00 p = 0.510) respectively. No statistically significant difference was found for the use of hydrotubation (pooled OR 1.4192, CI 0.4936 - 4.0804, I2= 67.3%, Tau2=0,57 p = 0.047), trigger (pooled OR 0.6649, CI 0.2422- 1.8257, I2= 74.7%, Tau2=0,3972, p = 0.047), hCG versus agonist trigger (pooled OR 1.1570, CI 0.7501- 1.7847, I2=0.0%, Tau2=0.00, p = 0.502), ultrasound guidance (pooled OR 2.119, CI 0.8289-5.3809, p = 0.437), double IUI (pooled OR 0.9718, CI 0.5721-1.6508, I2=0.0%, Tau2=0.00, p = 0.567), and bed rest (pooled OR 1.1459, CI 0.4789-2.7418, I2=0.0%, Tau2=0.00, p = 0.005). No eligible studies were found on tocolytic agents. Two studies were identified for the use of misoprostol but the authors do not report results on the outcomes of interest.

Limitations, reasons for caution

The risk of bias was considered as “high”/“some concerns” for all included studies and the overall quality of evidence is low. Included studies randomized both per woman and per cycle and significant differences were noticed in ovarian stimulation protocols. More well-designed RCTs are needed in order to reach sound conclusions.

Wider implications of the findings

This meta-analysis provides evidence that luteal phase support and endometrial scratch provide clinical benefit for IUI success rates. However, it is possible that different add-ons could benefit specific patient groups. There is place for more research in the field to explore the value of add-ons in relation to subfertility background.

Trial registration number

n/a

P-399 Effect of male body mass index on miscarriage rate following fertility treatment, a systematic review and meta-analysis

Study question

Is raised paternal body mass index (BMI) important for the miscarriage rate following assisted reproductive technology (ART)?

Summary answer

Based on the available evidence, raised male BMI is not associated with higher risk of miscarriage following ART.

What is known already

More than half women and men of reproductive age worldwide are overweight or obese.There is extensive literature exploring the importance of normal female BMI for reproductive outcomes. However, little attention has been given to male BMI for couples seeking fertility treatment [1] .The adverse effect of male obesity on sperm parameters including DNA damage is well documented and there is evidence suggesting that raised male BMI results in significant decrease in live birth rate following ART [2] .Furthermore, emerging evidence from human and animal studies demonstrates that paternal obesity can affect the future health of the offspring through epigenetic pathways[3].

Study design, size, duration

A computerized literature search was performed using EMBASE, MEDLINE, CINAHL and the Cochrane Central register of trials from database inception to November 2021. The aim was to explore the association between male BMI on miscarriage rate following ART. Reference lists of relevant studies were cross-checked. Only articles with full manuscripts available and published in English were included. Papers not relating to human subjects were excluded. All eligible studies were included (observational, prospective and retrospective studies).

Participants/materials, setting, methods

Included studies reported on couples undergoing ART for any indication using partner’s fresh sperm. Outcomes of interest were miscarriage rate and clinical pregnancy rate. Outcome data from each study were pooled and expressed as odds ratio (OR) with 95% confidence interval (CI) by using a random-effect model due to statistical heterogeneity in the outcome data[4]. Heterogeneity of treatment effects was evaluated using the I2 statistic to quantify the variation across studies caused by heterogeneity.

Main results and the role of chance

Abstract screening identified 197 relevant studies. After excluding duplicates, reviews and studies which did not fulfill the inclusion criteria, full manuscripts were accessed for 13 studies. Six studies were identified exploring the effect of male BMI on miscarriage following ART, two prospective and four retrospective. The quality of evidence was low using the GRADE framework. Meta-analysis was possible for three studies including 6793 couples undergoing ART. Outcomes were compared for male BMI < 25 kg/m2 versus BMI >/=25 kg/m2 . The pooled results did not show a statistically significant increase in miscarriage rate when the male partner was overweight or obese compare to normoweight (OR 1.32, 95% CI 0.82–2.1, P = 0.249). There was significant heterogeneity between the included studies (I 2 = 48.7%). There was no significant effect of male BMI on clinical pregnancy rate (OR 0.90, 95% CI 0.59–1.38, P = 0.637). For two of the remaining studies which could not be included in the meta-analysis due to missing data, the authors concluded that male BMI >25 was not associated with increased miscarriage risk whilst the most recent prospective study showed that high male BMI was associated with increased risk of chromosomal aberration-related miscarriages.

Limitations, reasons for caution

The number of the included studies and significant heterogeneity are the main limitations. It was not possible to account for important confounders such as age, subfertility diagnosis, type of stimulation and laboratory parameters including embryo grade. We grouped participants in two BMI categories therefore did not distinguish between overweight/obesity/morbid obesity.

Wider implications of the findings

Despite increasing evidence suggestive of adverse effect of raised male BMI on reproductive outcomes, there is limited literature exploring the impact on miscarriage rate following ART. More well-designed studies are needed for sound conclusions. Paternal characteristics, general health and preconception lifestyle should not be overlooked in the fertility consultation.

Trial registration number

not applicable

P-357 Predicting live birth following single euploid frozen blastocyst transfer - does the journey matter?

Study question

Does the number of available euploid embryos affect the success rate of the first single euploid frozen thawed embryo transfer (FTET)?

Summary answer

The live birth rate (LBR) following the first single euploid FTET is independent of the number of available euploid embryos.

What is known already

Preimplantation testing for aneuploidy (PGT-A) is a controversial laboratory technique as an embryo selection tool to increase LBR per transfer and reduce time to pregnancy. There have been many efforts to identify factors predictive of LBR following a euploid embryo transfer. The number of euploid embryos available is known to increase the expected cumulative LBR. However, there is lack of evidence whether the number of available euploid embryos is also significantly associated with chances of live birth for the first euploid FTET cycle. This is important for individualized patient counselling.

Study design, size, duration

A retrospective analysis of the first, single, euploid FTET of 506 women, between January 2015 to March 2020 following PGT-A was carried out. The indication for PGT-A was advanced maternal age, recurrent miscarriage or implantation failure. Embryos were biopsied on day 5 or 6 and tested with array comparative genetic hybridization or next generation sequencing. The FTET outcomes were compared based on the number of euploid embryos available.

Participants/materials, setting, methods

198 women had one euploid embryo available (group 1), 120 women had two (group 2) and 188 women had three or more (group 3). Baseline characteristics were analysed for all participants to adjust for confounders. Reproductive outcomes such as pregnancy rate (PR), clinical miscarriage rate and LBR were compared between the 3 groups for the first euploid FTET. Univariate analysis with Kruskal-Wallis H, chi-square and multivariate logistic regression was performed with SPSS.

Main results and the role of chance

Baseline characteristics were similar between the groups including age at egg collection (EC), body mass index, use of partner or donor sperm, method of insemination, number of ECs, endometrial preparation protocol and endometrial thickness prior to transfer. Women with three or more euploid embryos were younger at the time of EC compared to groups 1 and 2 (mean age 37 vs 39 and 38). The number of oocytes per EC (15 vs 11 and 11, p < 0.001), the total number of oocytes following batching cycles (20 vs 15 and 16, p < 0.001) and the number of blastocysts biopsied (8 vs 4 and 5, p < 0.001) were significantly higher for Group 3 vs Group 1 and Group 2, respectively. Group 1 embryos were less frequently of excellent and very good morphology and less frequently biopsied on day 5 vs 6.

Univariate analysis suggests Group 3 had statistically higher PR and LBR compared to Group 1, but similar miscarriage rate. However, when applying a multivariate logistic regression model, the LBR after first euploid FTET was independent of the number of euploid embryos available but was influenced by age at EC (p = 0.006) and biopsy day (Day 6 vs day 5, OR 0.53, 95% CI 0.33-0.85, p = 0.008).

Limitations, reasons for caution

The main limitation of this study is its retrospective nature performed in a single centre, and therefore vulnerable to bias and confounding factors that may have not been accounted for.

Wider implications of the findings

Our findings suggest that the success rate of the first euploid FTET does not depend on the number of euploid embryos but is influenced by the age at EC and the embryo stage at biopsy. This could be useful when counselling women with a low number of euploid embryos available.

Trial registration number

Not Applicable

P–625 Does endogenous progesterone play a role in unexplained infertility? A systematic review

Study question

Does endogenous progesterone play a role in unexplained infertility? A systematic review investigating the possibility of altered progesterone-mediated change leading to reduced endometrial receptivity in women with unexplained infertility.

Summary answer

The evidence suggests that many of the measurable actions of endogenous progesterone are reduced in women with unexplained infertility when compared with controls.

What is known already

Unexplained infertility is the diagnosis given to heterosexual couples who fail to conceive despite normal semen analysis, regular ovulation and patent tubes. The underlying pathology is likely to relate to embryonic failure to implant. Endometrial receptivity is largely mediated by luteal phase progesterone which controls many different molecular pathways involved in secretory transformation. It is possible that defective actions of progesterone could contribute to this condition. To date however, there is minimal published literature on the role of progesterone in unexplained infertility. We therefore felt it important to combine the results of all trials measuring progesterone-related outcomes in unexplained infertility.

Study design, size, duration

A systematic review was performed using standard Cochrane methodology. We searched Medline, Embase and CINAHL databases from inception to December 2020 and additionally hand-searched. The study was prospectively registered on Prospero (CRD42020141041). The search strategy was designed to identify all types of primary research published in English that investigated women with unexplained infertility and reported outcomes that relate to progesterone. Newcastle Ottawa Scoring and NHLBI assessment of bias scoring was performed.

Participants/materials, setting, methods

The study population was women with unexplained infertility. Included studies had no controls, fertile controls or controls with other diagnoses associated with subfertility. Outcomes were either upstream affecting progesterone production/release or receptor expression or downstream measuring results of progesterone-mediated processes. The results were summarised in a narrative review. Meta-analysis was not possible due to varying methodological heterogeneity.

Main results and the role of chance

36 studies were included. No difference was found in 18 studies in progesterone levels (serum, peritoneal and salivary) between women with unexplained infertility and control groups. Despite this, 32 of the 36 included studies demonstrated a significant difference between progesterone-mediated outcomes in the unexplained infertile and control groups.

5 ultrasound studies all reported increased resistance and decreased flow on doppler studies of uterine, ovarian and spiral arteries and reduced endometrial and sub-endometrial perfusion. No significant difference was found in luteal phase endometrial thickness in 2 studies.

Endometrial dating was reported by 11 studies. 8/11 studies reported significantly higher numbers (20–38%) of ‘out-of-phase’ endometrium in women with unexplained infertility compared with controls.

Endometrial biopsy results measuring different cell adhesion molecules, monoclonal antibodies and other molecules involved in endometrial transformation as well as expression of responsible genes and steroid hormone receptors were included. All the progesterone-mediated outcome measures listed above were reduced in unexplained infertile women except β3 integrin which reported contradictory results and SGK1 expression which was reported in 1 study. This trend towards support for the hypothesis may be more important than any individual finding. The quality of the included studies was variable and hence the strength of the recommendations moderate.

Limitations, reasons for caution

The number of studies measuring each outcome was limited. The study quality varied from good to poor. Methodological heterogeneity between studies prevented meta-analysis. The strength of the study however comes from the originality of the research, the variety of included outcomes and that 32/36 papers reported results supporting the hypothesis.

Wider implications of the findings: The findings of this systematic review support the need for larger, well designed research on this topic. If altered progesterone-mediated receptivity is implicated in unexplained infertility, it may be possible to offer other therapeutic interventions to improve outcomes as an alternative or adjunct to standard fertility treatment.

Trial registration number

NA

Elective oocyte cryopreservation for age-related fertility decline

PURPOSE

Women who pursue fertility at an advanced age are increasingly common. Family planning and sexual education have traditionally focused on contraception and prevention of sexually transmitted diseases. A focus should now also be placed on fertility awareness and fertility preservation. This manuscript aims to give an update on the existing evidence around elective oocyte cryopreservation, also highlighting the need for fertility education and evidence-based, individualized counselling.

METHODS

A thorough electronic search was performed from the start of databases to March 2020 aiming to summarize the existing evidence around elective egg freezing, the logic behind its use, patient counselling and education, success rates and risks involved, regulation, cost-effectiveness, current status and future perspectives.

RESULTS

Clinician-led counselling regarding reproductive aging and fertility preservation is often overlooked. Elective oocyte cryopreservation is not a guarantee of live birth, and the answer regarding cost-effectiveness needs to be individualized. The existing studies on obstetric and perinatal outcomes following the use of egg freezing are, until now, reassuring. Constant monitoring of short-term and long-term outcomes, uniform regulation and evidence-based, individualized counselling is of paramount importance.

CONCLUSIONS

Elective oocyte cryopreservation is one of the most controversial aspects of the world of assisted reproduction, and a lot of questions remain unanswered. However, women today do have this option which was not available in the past. Elective oocyte cryopreservation for age-related fertility decline should be incorporated in women's reproductive options to ensure informed decisions and reproductive autonomy.

MT-ND5 Mutation Exhibits Highly Variable Neurological Manifestations at Low Mutant Load

Mutations in the m.13094T>C MT-ND5 gene have been previously described in three cases of Leigh Syndrome (LS). In this retrospective, international cohort study we identified 20 clinically affected individuals (13 families) and four asymptomatic carriers. Ten patients were deceased at the time of analysis (median age of death was 10years (range: 5·4months-37years, IQR=17·9years). Nine patients manifested with LS, one with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS), and one with Leber hereditary optic neuropathy. The remaining nine patients presented with either overlapping syndromes or isolated neurological symptoms. Mitochondrial respiratory chain activity analysis was normal in five out of ten muscle biopsies. We confirmed maternal inheritance in six families, and demonstrated marked variability in tissue segregation, and phenotypic expression at relatively low blood mutant loads. Neuropathological studies of two patients manifesting with LS/MELAS showed prominent capillary proliferation, microvacuolation and severe neuronal cell loss in the brainstem and cerebellum, with conspicuous absence of basal ganglia involvement. These findings suggest that whole mtDNA genome sequencing should be considered in patients with suspected mitochondrial disease presenting with complex neurological manifestations, which would identify over 300 known pathogenic variants including the m.13094T>C.

Autoreactive IgG elicited in mice by the non-dominant but pathogenic thyroglobulin peptide (2495-2511): implications for thyroid autoimmunity

We have previously shown that mice challenged with the rat thyroglobulin (Tg) peptide TgP1 (corresponding to aa 2495-2511 of human Tg) develop experimental autoimmune thyroiditis (EAT) and produce IgG antibodies that cross-react with Tg from various species. It was not clear, however, whether such antibodies were TgP1-specific or were induced secondarily--i.e. by autologous Tg released from the destroyed gland--and therefore directed to determinants other than TgP1. In this study we describe that, 5 weeks after priming with TgP1, the binding of serum IgG on native Tg is completely inhibited by free peptide, suggesting lack of recognition of other determinants on mouse Tg (mTg). In addition, TgP1-induced but not mTg-induced IgG bound better to heat-denatured than intact mTg, a result compatible with the recognition of a linear epitope by the peptide-induced antibodies. Comparison of the IgG subclass distribution among mTg-induced versus TgP1-induced IgG did not reveal qualitative differences, since all subclasses were represented in the order IgG1 > IgG2b > IgG2a > IgG3. Finally, TgP1-specific IgG reacted strongly with the follicular colloid in sections of normal thyroids, indicating the potential to bind to native Tg in vivo. These data: (i) highlight TgP1 as the only, so far, Tg sequence known to generate both EAT and Tg-reactive IgG in mice; and (ii) do not provide evidence for an amplification of the Tg-specific IgG response through the involvement of endogenous autoantigen in EAT.

Distinct genetic pattern of mouse susceptibility to thyroiditis induced by a novel thyroglobulin peptide

Experimental autoimmune thyroiditis (EAT), induced by thyroglobulin (Tg) and adjuvant, is major histocompatibility complex-controlled and dependent on Tg-reactive T cells, but the immunopathogenic T-cell epitopes on Tg remain mostly undefined. We report here the thyroiditogenicity of a novel rat Tg peptide (TgP2; corresponding to human Tg amino acids 2695-2713), identified by algorithms as a site of putative T-cell epitope(s). TgP2 causes EAT in SJL (H-2s) but not in C3H or B10.BR (H-2k), BALB/c (H-2d), and B10 (H-2b) mice. This reveals a new genetic pattern of EAT susceptibility, since H-2k mice are known to be high responders (susceptible) after Tg challenge. Following in vivo priming with TgP2, T cells from only SJL mice proliferated significantly and consistently to TgP2 in vitro, whereas TgP2-specific IgG was observed in all strains tested. Adoptive transfer of TgP2-primed SJL lymph node cells to naive syngeneic recipients induced a pronounced mononuclear infiltration of the thyroid, which was more extensive than that observed after direct peptide challenge. TgP2 is non-immunodominant, since priming of SJL mice with rTg did not consistently elicit T-cell responses to TgP2 in vitro and a TgP2-specific T-cell hybridoma did not respond to antigen presenting cells pulsed with rTg. The data support the notion that Tg epitopes need not be either iodinated or immunodominant in order to cause severe thyroiditis and that the genetic pattern of the disease they induce can be distinct from that of Tg-mediated EAT.

Identification of a thyroiditogenic sequence within the thyroglobulin molecule

Thyroglobulin (Tg)-specific T cells are important in the induction of experimental autoimmune thyroiditis (EAT), but the nature and the number of the Tg T cell epitopes involved in the disease process are unknown. Through the use of computerized algorithms that search for putative T cell epitopes, a 17-mer peptide (TgP1) was identified within the known portion of the rat Tg sequence (corresponding to amino acids 2495 to 2511 of the human Tg sequence) that induced strong mononuclear cell infiltration of the thyroid in classic EAT-susceptible murine strains such as SJL, C3H, and B10.BR and low or undetectable infiltration in EAT-resistant strains such as BALB/c and B10. TgP1 appears to be phylogenetically conserved since it is completely homologous to its bovine counterpart and differs at a single amino acid position from its human analogue. After priming with TgP1 in vivo, significant proliferative T cell responses to TgP1 in vitro were observed only with lymphocytes from susceptible (high responder) strains, thus correlating proliferative capacity with EAT induction. TgP1-primed T cells did not respond to intact mouse Tg (MTg) or rat Tg in vitro and, conversely, T cells primed in vivo with MTg or rat Tg did not respond to TgP1 in culture, suggesting that TgP1 is comprised of non-immunodominant T cell determinants. TgP1 was defined as a serologically nonimmunodominant epitope as well, since in vivo priming of all strains with MTg led to strong MTg-specific IgG responses but no TgP1-specific responses in ELISA assays. This was not due to lack of immunogenic B cell determinants on TgP1, however, because peptide challenge of EAT-susceptible strains elicited TgP1-specific IgG that also cross-reacted with MTg and rat, human, bovine, and porcine Tg. The data demonstrate that TgP1 delineates nonimmunodominant but highly immunogenic determinants at both the T and B cell level, which may play an important role in the development of autoimmune thyroiditis.

Identification of a thyroiditogenic sequence within the thyroglobulin molecule

Thyroglobulin (Tg)-specific T cells are important in the induction of experimental autoimmune thyroiditis (EAT), but the nature and the number of the Tg T cell epitopes involved in the disease process are unknown. Through the use of computerized algorithms that search for putative T cell epitopes, a 17-mer peptide (TgP1) was identified within the known portion of the rat Tg sequence (corresponding to amino acids 2495 to 2511 of the human Tg sequence) that induced strong mononuclear cell infiltration of the thyroid in classic EAT-susceptible murine strains such as SJL, C3H, and B10.BR and low or undetectable infiltration in EAT-resistant strains such as BALB/c and B10. TgP1 appears to be phylogenetically conserved since it is completely homologous to its bovine counterpart and differs at a single amino acid position from its human analogue. After priming with TgP1 in vivo, significant proliferative T cell responses to TgP1 in vitro were observed only with lymphocytes from susceptible (high responder) strains, thus correlating proliferative capacity with EAT induction. TgP1-primed T cells did not respond to intact mouse Tg (MTg) or rat Tg in vitro and, conversely, T cells primed in vivo with MTg or rat Tg did not respond to TgP1 in culture, suggesting that TgP1 is comprised of non-immunodominant T cell determinants. TgP1 was defined as a serologically nonimmunodominant epitope as well, since in vivo priming of all strains with MTg led to strong MTg-specific IgG responses but no TgP1-specific responses in ELISA assays. This was not due to lack of immunogenic B cell determinants on TgP1, however, because peptide challenge of EAT-susceptible strains elicited TgP1-specific IgG that also cross-reacted with MTg and rat, human, bovine, and porcine Tg. The data demonstrate that TgP1 delineates nonimmunodominant but highly immunogenic determinants at both the T and B cell level, which may play an important role in the development of autoimmune thyroiditis.