Control of human cytomegalovirus replication by liver resident natural killer cells.

Natural killer cells are considered to be important for control of human cytomegalovirus- a major pathogen in immune suppressed transplant patients. Viral infection promotes the development of an adaptive phenotype in circulating natural killer cells that changes their anti-viral function. In contrast, less is understood how natural killer cells that reside in tissue respond to viral infection. Here we show natural killer cells resident in the liver have an altered phenotype in cytomegalovirus infected individuals and display increased anti-viral activity against multiple viruses in vitro and identify and characterise a subset of natural killer cells responsible for control. Crucially, livers containing natural killer cells with better capacity to control cytomegalovirus replication in vitro are less likely to experience viraemia post-transplant. Taken together, these data suggest that virally induced expansion of tissue resident natural killer cells in the donor organ can reduce the chance of viraemia post-transplant.

Cardiorespiratory Optimisation By Arteriovenous fistula Ligation after renal Transplantation (COBALT): study protocol for a multicentre randomised interventional feasibility trial.

INTRODUCTION: Cardiovascular events are a major cause of mortality following successful kidney transplantation.Arteriovenous fistulas (AVFs) are considered the best option for haemodialysis, but may contribute to this excess mortality because they promote adverse cardiac remodelling and ventricular hypertrophy. This raises the question whether recipients with a well-functioning kidney transplant should undergo elective AVF ligation. METHODS AND ANALYSIS: The COBALT feasibility study is a multicentre interventional randomised controlled trial (RCT) that will randomise renal transplant patients with stable graft function and a working AVF on a 1:1 basis to standard care (continued conservative management) or to AVF ligation. All patients will perform cardiopulmonary exercise testing (CPET) on recruitment and 6 months later. Daily functioning and quality of life will be additionally assessed by questionnaire completion and objective measure of physical activity. The primary outcome-the proportion of approached patients who complete the study (incorporating rates of consent, receipt of allocated intervention and completion of both CPETs without withdrawal)-will determine progression to a full-scale RCT. Design of the proposed RCT will be informed by an embedded qualitative assessment of participant and healthcare professional involvement. ETHICS AND DISSEMINATION: This study has been approved by the East Midlands-Derby Research Ethics Committee (22/EM/0002) and the Health Research Authority. The results of this work will be disseminated academically through presentation at national and international renal meetings and via open access, peer-reviewed outputs. Existing networks of renal patient groups will also be used to disseminate the study findings to other key stakeholders. TRIAL REGISTRATION NUMBER: ISRCTN49033491.

Passenger donor lymphocytes: To affinity and beyond.

Graft-versus-host recognition by passenger donor lymphocytes within organ transplants can trigger host alloantibody production (Charmetant et al.).

Kidney Transplantation From Deceased Donors With Vaccine-induced Immune Thrombocytopenia and Thrombosis: An Updated Analysis of the UK Experience.

BACKGROUND: The emergence and attendant mortality of vaccine-induced immune thrombocytopenia and thrombosis (VITT) as a consequence of vaccination against severe acute respiratory syndrome coronavirus 2 have resulted in some patients with VITT being considered as deceased organ donors. Outcomes after kidney transplantation in this context are poorly described. Because the disease seems to be mediated by antiplatelet factor 4 antibodies, there is a theoretical risk of transmission via passenger leukocytes within the allograft. METHODS: We analyzed the experience of kidney transplantation from donors with VITT in the United Kingdom between January and June 2021. We followed-up all recipients of kidney-only transplants from donors with VITT to detect major postoperative complications or features of disease transmission and assess graft survival and function. RESULTS: There were 16 kidney donors and 30 single kidney transplant recipients in our study period. Of 11 preimplantation biopsies, 4 showed widespread glomerular microthrombi. After a median of 5 mo, patient and graft survival were 97% and 90%, respectively. The median 3-mo estimated glomerular filtration rate was 51 mL/min/1.73 m 2 . Two recipients had detectable antiplatelet factor 4 antibodies but no evidence of clinical disease after transplantation. Major hemorrhagic complications occurred in 3 recipients, all of whom had independent risk factors for bleeding, resulting in the loss of 2 grafts. The involvement of VITT could not be completely excluded in one of these cases. CONCLUSIONS: The UK experience to date shows that favorable outcomes are possible after kidney transplantation from donors with VITT but highlights the need for ongoing vigilance for donor-related complications in these patients.

Severity of COVID-19 after Vaccination among Hemodialysis Patients: An Observational Cohort Study.

BACKGROUND AND OBJECTIVES: Patients receiving hemodialysis are at high risk from coronavirus disease 2019 (COVID-19) and demonstrate impaired immune responses to vaccines. There have been several descriptions of their immunologic responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination, but few studies have described the clinical efficacy of vaccination in patients on hemodialysis. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: In a multicenter observational study of the London hemodialysis population undergoing surveillance PCR testing during the period of vaccine rollout with BNT162b2 and AZD1222, all of those positive for SARS-CoV-2 were identified. Clinical outcomes were analyzed according to predictor variables, including vaccination status, using a mixed effects logistic regression model. Risk of infection was analyzed in a subgroup of the base population using a Cox proportional hazards model with vaccination status as a time-varying covariate. RESULTS: SARS-CoV-2 infection was identified in 1323 patients of different ethnicities (Asian/other, 30%; Black, 38%; and White, 32%), including 1047 (79%) unvaccinated, 86 (7%) after first-dose vaccination, and 190 (14%) after second-dose vaccination. The majority of patients had a mild course; however, 515 (39%) were hospitalized, and 172 (13%) died. Older age, diabetes, and immune suppression were associated with greater illness severity. In regression models adjusted for age, comorbidity, and time period, prior two-dose vaccination was associated with a 75% (95% confidence interval, 56 to 86) lower risk of admission and 88% (95% confidence interval, 70 to 95) fewer deaths compared with unvaccinated patients. No loss of protection was seen in patients over 65 years or with increasing time since vaccination, and no difference was seen between vaccine types. CONCLUSIONS: These data demonstrate a substantially lower risk of severe COVID-19 after vaccination in patients on dialysis who become infected with SARS-CoV-2.

Outcome and effect of vaccination in SARS-CoV-2 Omicron infection in hemodialysis patients: a cohort study.

BACKGROUND: Hemodialysis patients are at high risk of Covid-19, though vaccination has significant efficacy in preventing and reducing the severity of infection. Little information is available on disease severity and vaccine efficacy since the dissemination of the Omicron variant. METHODS: In a multi-center study, during a period of the epidemic driven by the Omicron variant, all hemodialysis patients positive for SARS-CoV-2 were identified. Outcomes were analyzed according to predictor variables including vaccination status. Risk of infection was analyzed using a Cox proportional hazards model. RESULTS: SARS-CoV-2 infection was identified in 1126 patients including 200 (18%) unvaccinated, 56 (5%) post first dose, 433 (38%) post second dose, and 437 (39%) at least 7 days beyond their third dose. The majority of patients had a mild course but 160 (14%) were hospitalized and 28 (2%) died. In regression models adjusted for age and comorbidity, two-dose vaccination was associated with a 39% (95%CI: 2%-62%) reduction in admissions, but third doses provided additional protection, with a 51% (95%CI: 25%-69%) further reduction in admissions. Among 1265 patients at risk at the start of the observation period, SARS-CoV-2 infection was observed in 211 (17%). Two-dose vaccination was associated with a 41% (95%CI: 3%-64%) reduction in the incidence of infection, with no clear additional effect provided by third doses. CONCLUSIONS: These data demonstrate lower incidence of SARS-CoV-2 infection after vaccination in dialysis patients during an Omicron dominant period of the epidemic. Among those developing infection, severe illness was less common with prior vaccination, particularly after third vaccine doses.

Anaesthesia Choice for Creation of Arteriovenous Fistula (ACCess) study protocol : a randomised controlled trial comparing primary unassisted patency at 1 year of primary arteriovenous fistulae created under regional compared to local anaesthesia.

INTRODUCTION: Arteriovenous fistulae (AVF) are the 'gold standard' vascular access for haemodialysis. Universal usage is limited, however, by a high early failure rate. Several small, single-centre studies have demonstrated better early patency rates for AVF created under regional anaesthesia (RA) compared with local anaesthesia (LA). The mechanistic hypothesis is that the sympathetic blockade associated with RA causes vasodilatation and increased blood flow through the new AVF. Despite this, considerable variation in practice exists in the UK. A high-quality, adequately powered, multicentre randomised controlled trial (RCT) is required to definitively inform practice. METHODS AND ANALYSIS: The Anaesthesia Choice for Creation of Arteriovenous Fistula (ACCess) study is a multicentre, observer-blinded RCT comparing primary radiocephalic/brachiocephalic AVF created under regional versus LA. The primary outcome is primary unassisted AVF patency at 1 year. Access-specific (eg, stenosis/thrombosis), patient-specific (including health-related quality of life) and safety secondary outcomes will be evaluated. Health economic analysis will also be undertaken. ETHICS AND DISSEMINATION: The ACCess study has been approved by the West of Scotland Research and ethics committee number 3 (20/WS/0178). Results will be published in open-access peer-reviewed journals within 12 months of completion of the trial. We will also present our findings at key national and international renal and anaesthetic meetings, and support dissemination of trial outcomes via renal patient groups. TRIAL REGISTRATION NUMBER: ISRCTN14153938. SPONSOR: NHS Greater Glasgow and Clyde GN19RE456, Protocol V.1.3 (8 May 2021), REC/IRAS ID: 290482.

Kidney transplantation and patients who decline SARS-CoV-2 vaccination: an ethical framework.

As SARS-CoV-2 vaccines have started to be rolled out, a key question facing transplant units has been whether listing for transplantation should be contingent on recipients having received a vaccine. We aimed to provide an ethical framework when considering potential transplant candidates who decline vaccination. We convened a working group comprising transplant professionals, lay members and patients and undertook a literature review and consensus process. This group's work was also informed by discussions in two hospital clinical ethics committees. We have reviewed arguments for and against mandating vaccination prior to listing for kidney transplantation and considered some practical difficulties which may be associated with a policy of mandated vaccination. Rather than requiring that all patients must receive the SARS-CoV-2 vaccine prior to transplant listing, we recommend considering vaccination status as one of a number of SARS-CoV-2-related risk factors in relation to transplant listing. Transplant units should engage in individualised risk-benefit discussions with patients, avoid the language of mandated treatments and strongly encourage uptake of the vaccine in all patient groups, using tailor-made educational initiatives.

Neutralizing Antibody Responses After SARS-CoV-2 Infection in End-Stage Kidney Disease and Protection Against Reinfection.

INTRODUCTION: Patients with end-stage kidney disease (ESKD) represent a vulnerable group with multiple risk factors that are associated with poor outcomes after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Despite established susceptibility to infectious complications and the importance of humoral immunity in protection against SARS-CoV-2, few studies have investigated the humoral immune response to SARS-CoV-2 within this population. Here, we evaluate the seroprevalence of SARS-CoV-2 in patients awaiting renal transplantation and determine whether seroconverted patients with ESKD have durable and functional neutralizing activity against SARS-CoV-2. METHODS: Serum samples were obtained from 164 patients with ESKD by August 2020. Humoral immune responses were evaluated by SARS-CoV-2 spike S1 subunit and nucleoprotein semiquantitative enzyme-linked immunosorbent assay (ELISA) and SARS-CoV-2 spike pseudotype neutralization assay. RESULTS: All patients with ESKD with reverse-transcriptase polymerase chain reaction (RT-PCR)-confirmed infection (n = 17) except for 1 individual seroconverted against SARS-CoV-2. Overall seroprevalence (anti-S1 and/or anti-N IgG) was 36% and was higher in patients on hemodialysis (44.2%). A total of 35.6% of individuals who seroconverted were asymptomatic. Seroconversion in the absence of a neutralizing antibody (nAb) titer was observed in 12 patients, all of whom were asymptomatic. Repeat measurements at a median of 93 days from baseline sampling revealed that most individuals retained detectable responses although a significant drop in S1, N and nAb titers was observed. CONCLUSION: Patients with ESKD, including those who develop asymptomatic disease, routinely seroconvert and produce detectable nAb titers against SARS-CoV-2. Although IgG levels wane over time, the neutralizing antibodies remain detectable in most patients, suggesting some level of protection is likely maintained, particularly in those who originally develop stronger responses.

Renal transplant outcomes in amyloidosis.

BACKGROUND: Outcomes after renal transplantation have traditionally been poor in systemic amyloid A (AA) amyloidosis and systemic light chain (AL) amyloidosis, with high mortality and frequent recurrent disease. We sought to compare outcomes with matched transplant recipients with autosomal dominant polycystic kidney disease (ADPKD) and diabetic nephropathy (DN), and identify factors predictive of outcomes. METHODS: We performed a retrospective cohort study of 51 systemic AL and 48 systemic AA amyloidosis patients undergoing renal transplantation. Matched groups were generated by propensity score matching. Patient and death-censored allograft survival were compared via Kaplan-Meier survival analyses, and assessment of clinicopathological features predicting outcomes via Cox proportional hazard analyses. RESULTS: One-, 5- and 10-year death-censored unadjusted graft survival was, respectively, 94, 91 and 78% for AA amyloidosis, and 98, 93 and 93% for AL amyloidosis; median patient survival was 13.1 and 7.9 years, respectively. Patient survival in AL and AA amyloidosis was comparable to DN, but poorer than ADPKD [hazard ratio (HR) = 3.12 and 3.09, respectively; P < 0.001]. Death-censored allograft survival was comparable between all groups. In AL amyloidosis, mortality was predicted by interventricular septum at end diastole (IVSd) thickness >12 mm (HR = 26.58; P = 0.03), while survival was predicted by haematologic response (very good partial or complete response; HR = 0.07; P = 0.018). In AA amyloidosis, recurrent amyloid was associated with elevated serum amyloid A concentration but not with outcomes. CONCLUSIONS: Renal transplantation outcomes for selected patients with AA and AL amyloidosis are comparable to those with DN. In AL amyloidosis, IVSd thickness and achievement of deep haematologic response pre-transplant profoundly impact patient survival.

Mortality Rates in Transplant Recipients and Transplantation Candidates in a High-prevalence COVID-19 Environment.

BACKGROUND: The risk of COVID-19 infection in transplant recipients (TRs) is unknown. Patients on dialysis may be exposed to greater risk of infection due to an inability to isolate. Consideration of these competing risks is important before restarting suspended transplant programs. This study compared outcomes in kidney and kidney/pancreas TRs with those on the waiting list, following admission with COVID-19 in a high-prevalence region. METHODS: Audit data from all 6 London transplant centers were amalgamated. Demographic and laboratory data were collected and outcomes included mortality, intensive care (ITU) admission, and ventilation. Adult patients who had undergone a kidney or kidney/pancreas transplant, and those active on the transplant waiting list at the start of the pandemic were included. RESULTS: One hundred twenty-one TRs and 52 waiting list patients (WL) were admitted to hospital with COVID-19. Thirty-six TR died (30%), while 14 WL patients died (27% P = 0.71). There was no difference in rates of admission to ITU or ventilation. Twenty-four percent of TR required renal replacement therapy, and 12% lost their grafts. Lymphocyte nadir and D-dimer peak showed no difference in those who did and did not die. No other comorbidities or demographic factors were associated with mortality, except for age (odds ratio of 4.3 [95% CI 1.8-10.2] for mortality if aged over 60 y) in TR. CONCLUSIONS: TRs and waiting list patients have similar mortality rates after hospital admission with COVID-19. Mortality was higher in older TRs. These data should inform decisions about transplantation in the COVID era.

Humoral autoimmunity after solid organ transplantation: Germinal ideas may not be natural.

Non-HLA antibody responses following solid organ transplantation have become increasingly emphasised, with several large clinical series suggesting that such responses contribute to late graft failure. Many of the responses described recognise both recipient and donor moieties of the target antigen and thus represent auto-, rather than allo-immunity. Within this rapidly evolving field, many questions remain unanswered: what triggers the response; how innate and adaptive humoral autoimmunity integrate; and most pressingly, how autoimmunity contributes to graft damage and its relationship to other effector mechanisms of graft rejection. This review summarises recent clinical and experimental studies of humoral autoimmunity in transplant rejection, and considers some of the answers to these questions.

Surveillance arterioveNous fistulAs using ultRasound (SONAR) trial in haemodialysis patients: a study protocol for a multicentre observational study.

INTRODUCTION: Arteriovenous fistulas (AVFs) are considered the best and safest modality for providing haemodialysis in patients with end-stage renal disease. Only 20% of UK centres achieve the recommended 80% target for achieving dialysis of the prevalent dialysis population via permanent access (as opposed to a central venous catheter). This is partly due to the relatively poor maturation rate of newly created fistulas, with as many as 50% of fistulas failing to mature.The Surveillance Of arterioveNous fistulAe using ultRasound study will examine whether a protocolised programme of Doppler ultrasound (US) surveillance can identify, early after creation, potentially correctable problems in those AVFs that subsequently fail to mature. METHODS AND ANALYSIS: This is a multicentre observational study that will assess newly created AVFs by Doppler US performed at 2, 4, 6 and 10 weeks after creation. The primary outcome measure will be primary fistula patency at week 10. Secondary outcome measures include: successful use of the fistula; clinical suitability for dialysis; creation of new fistula or radiological salvage; fistula thrombosis; secondary fistula patency rate and patient acceptability. ETHICS AND DISSEMINATION: The study has been approved by the Cambridgeshire and Hertfordshire Research Ethics Committee and by the Health Research Authority (REC 18/EE/0234). The results generated from this work will be published as open access, within 3 years of trial commencement. We will also present our findings at key national/international renal meetings, as well as support volunteers at renal patient groups to disseminate the trial outcome. TRIAL REGISTRATION NUMBER: ISRCTN36033877.

Data regarding transplant induced germinal center humoral autoimmunity.

This data is related to the research article entitled "Germinal center humoral autoimmunity independently mediates progression of allograft vasculopathy" (Harper et al., 2016) [2]. The data presented here focuses on the humoral autoimmune response triggered by transferred allogeneic CD4 T cells and includes details on: (a) the recipient splenic germinal center (GC) response; (b) augmentation of humoral autoimmunity and accelerated heart allograft rejection following transplantation from donors primed against recipient; (c) flow cytometric analysis of donor and recipient CD4 T cells for signature markers of T follicular helper cell differentiation; (d) in vitro donor endothelial cell migration in response to column purified autoantibody from recipient sera; (e) analysis of development of humoral responses in recipients following adoptive transfer of donor CD4 T cells and; (f) the development of humoral autoimmunity in mixed haematopoietic chimeric mice.

Germinal center humoral autoimmunity independently mediates progression of allograft vasculopathy.

The development of humoral autoimmunity following organ transplantation is increasingly recognised, but of uncertain significance. We examine whether autoimmunity contributes independently to allograft rejection. In a MHC class II-mismatched murine model of chronic humoral rejection, we report that effector antinuclear autoantibody responses were initiated upon graft-versus-host allorecognition of recipient B cells by donor CD4 T-cells transferred within heart allografts. Consequently, grafts were rejected more rapidly, and with markedly augmented autoantibody responses, upon transplantation of hearts from donors previously primed against recipient. Nevertheless, rejection was dependent upon recipient T follicular helper (TFH) cell differentiation and provision of cognate (peptide-specific) help for maintenance as long-lived GC reactions, which diversified to encompass responses against vimentin autoantigen. Heart grafts transplanted into stable donor/recipient mixed haematopoietic chimeras, or from parental strain donors into F1 recipients (neither of which can trigger host adaptive alloimmune responses), nevertheless provoked GC autoimmunity and were rejected chronically, with rejection similarly dependent upon host TFH cell differentiation. Thus, autoantibody responses contribute independently of host adaptive alloimmunity to graft rejection, but require host TFH cell differentiation to maintain long-lived GC responses. The demonstration that one population of helper CD4 T-cells initiates humoral autoimmunity, but that a second population of TFH cells is required for its maintenance as a GC reaction, has important implications for how autoimmune-related phenomena manifest.

Prolongation of allograft survival by passenger donor regulatory T cells.

Tissue resident lymphocytes are present within many organs, and are presumably transferred at transplantation, but their impact on host immunity is unclear. Here, we examine whether transferred donor natural regulatory CD4 T cells (nT-regs) inhibit host alloimmunity and prolong allograft survival. Transfer of donor-strain lymphocytes was first assessed by identifying circulating donor-derived CD4 T cells in 21 consecutive human lung transplant recipients, with 3 patterns of chimerism apparent: transient, intermediate, and persistent (detectable for up to 6 weeks, 6 months, and beyond 1 year, respectively). The potential for transfer of donor nT-regs was then confirmed by analysis of leukocyte filters recovered from ex vivo normothermic perfusion circuits of human kidneys retrieved for transplantation. Finally, in a murine model of cardiac allograft vasculopathy, depletion of donor CD4 nT-regs before organ recovery resulted in markedly accelerated heart allograft rejection and augmented host effector antibody responses. Conversely, adoptive transfer or purified donor-strain nT-regs inhibited host humoral immunity and prolonged allograft survival, and more effectively so than following administration of recipient nT-regs. In summary, following transplantation, passenger donor-strain nT-regs can inhibit host adaptive immune responses and prolong allograft survival. Isolated donor-derived nT-regs may hold potential as a cellular therapy to improve transplant outcomes.

Germinal Center Alloantibody Responses Mediate Progression of Chronic Allograft Injury.

Different profiles of alloantibody responses are observed in the clinic, with those that persist, often despite targeted treatment, associated with poorer long-term transplant outcomes. Although such responses would suggest an underlying germinal center (GC) response, the relationship to cellular events within the allospecific B cell population is unclear. Here we examine the contribution of germinal center (GC) humoral alloimmunity to chronic antibody mediated rejection (AMR). A murine model of chronic AMR was developed in which T cell deficient (Tcrbd-/-) C57BL/6 recipients were challenged with MHC-mismatched BALB/c heart allografts and T cell help provided by reconstituting with 103 "TCR75" CD4 T cells that recognize self-restricted allopeptide derived from the H-2Kd MHC class I alloantigen. Reconstituted recipients developed Ig-switched anti-Kd alloantibody responses that were slow to develop, but long-lived, with confocal immunofluorescence and flow cytometric characterization of responding H-2Kd-allospecific B cells confirming persistent splenic GC activity. This was associated with T follicular helper (TFH) cell differentiation of the transferred TCR75 CD4 T cells. Heart grafts developed progressive allograft vasculopathy, and were rejected chronically (MST 50 days), with explanted allografts displaying features of humoral vascular rejection. Critically, late alloantibody responses were abolished, and heart grafts survived indefinitely, in recipients reconstituted with Sh2d1a-/- TCR75 CD4 T cells that were genetically incapable of providing TFH cell function. The GC response was associated with affinity maturation of the anti-Kd alloantibody response, and its contribution to progression of allograft vasculopathy related principally to secretion of alloantibody, rather than to enhanced alloreactive T cell priming, because grafts survived long-term when B cells could present alloantigen, but not secrete alloantibody. Similarly, sera sampled at late time points from chronically-rejecting recipients induced more vigorous donor endothelial responses in vitro than sera sampled earlier after transplantation. In summary, our results suggest that chronic AMR and progression of allograft vasculopathy is dependent upon allospecific GC activity, with critical help provided by TFH cells. Clinical strategies that target the TFH cell subset may hold therapeutic potential. This work is composed of two parts, of which this is Part II. Please read also Part I: Alsughayyir et al., 2019.

Relative Frequencies of Alloantigen-Specific Helper CD4 T Cells and B Cells Determine Mode of Antibody-Mediated Allograft Rejection.

Humoral alloimmunity is now recognized as a major determinant of transplant outcome. MHC glycoprotein is considered a typical T-dependent antigen, but the nature of the T cell alloresponse that underpins alloantibody generation remains poorly understood. Here, we examine how the relative frequencies of alloantigen-specific B cells and helper CD4 T cells influence the humoral alloimmune response and how this relates to antibody-mediated rejection (AMR). An MHC-mismatched murine model of cardiac AMR was developed, in which T cell help for alloantibody responses in T cell deficient (Tcrbd-/-) C57BL/6 recipients against donor H-2Kd MHC class I alloantigen was provided by adoptively transferred "TCR75" CD4 T cells that recognize processed H-2Kd allopeptide via the indirect-pathway. Transfer of large numbers (5 × 105) of TCR75 CD4 T cells was associated with rapid development of robust class-switched anti-H-2Kd humoral alloimmunity and BALB/c heart grafts were rejected promptly (MST 9 days). Grafts were not rejected in T and B cell deficient Rag2-/- recipients that were reconstituted with TCR75 CD4 T cells or in control (non-reconstituted) Tcrbd-/- recipients, suggesting that the transferred TCR75 CD4 T cells were mediating graft rejection principally by providing help for effector alloantibody responses. In support, acutely rejecting BALB/c heart grafts exhibited hallmark features of acute AMR, with widespread complement C4d deposition, whereas cellular rejection was not evident. In addition, passive transfer of immune serum from rejecting mice to Rag2-/- recipients resulted in eventual BALB/c heart allograft rejection (MST 20 days). Despite being long-lived, the alloantibody responses observed at rejection of the BALB/c heart grafts were predominantly generated by extrafollicular foci: splenic germinal center (GC) activity had not yet developed; IgG secreting cells were confined to the splenic red pulp and bridging channels; and, most convincingly, rapid graft rejection still occurred when recipients were reconstituted with similar numbers of Sh2d1a-/- TCR75 CD4 T cells that are genetically incapable of providing T follicular helper cell function for generating GC alloimmunity. Similarly, alloantibody responses generated in Tcrbd-/- recipients reconstituted with smaller number of wild-type TCR75 CD4 T cells (103), although long-lasting, did not have a discernible extrafollicular component, and grafts were rejected much more slowly (MST 50 days). By modeling antibody responses to Hen Egg Lysozyme protein, we confirm that a high ratio of antigen-specific helper T cells to B cells favors development of the extrafollicular response, whereas GC activity is favored by a relatively high ratio of B cells. In summary, a relative abundance of helper CD4 T cells favors development of strong extrafollicular alloantibody responses that mediate acute humoral rejection, without requirement for GC activity. This work is composed of two parts, of which this is Part I. Please read also Part II: Chhabra et al., 2019.