Triple-negative breast cancer (TNBC) patients with brain metastases have a poor prognosis and limited treatment options. Preclinical and clinical evidence suggests that radiotherapy may act synergistically with immune checkpoint inhibitors. We conducted an open-label, single-arm, phase II study of atezolizumab plus stereotactic radiosurgery (SRS) in metastatic TNBC patients with brain metastases. The primary endpoint was progression-free survival (PFS) according to the Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) bi-compartmental model. Secondary endpoints included extracranial objective response rate, overall survival (OS), and safety and tolerability. A safety run-in analysis for dose-limiting toxicity (DLT) was performed after the first 6 patients were enrolled and completed the assessment period. Six patients were enrolled into the safety run-in phase between May 11, 2018 and October 24, 2019. No DLTs were observed, but the study was closed early due to slow accrual. Patients received a median of 2 atezolizumab cycles (range: 2—16), and SRS was administered to all 6 patients. Treatment-related adverse events (TRAEs) occurred in 4 participants (66.7 https://www.clinicaltrials.gov NCT03483012. Trial Open to Accrual: 05/01/2018.

Representativeness of Study Participants

IMPORTANCE:Brain metastases portend poor prognosis in patients with metastatic breast cancer (MBC). Designing treatment and prevention clinical trials requires knowledge of brain metastases incidence with each line of therapy. OBJECTIVES:We assessed the prevalence and cumulative incidence of brain metastases in a large MBC patient cohort by subtype and line of therapy, and the impact of HER2-low expression on prevalence. DESIGN, SETTING AND OUTCOMES:We analyzed brain metastases prevalence in patients with MBC in a nationwide electronic health record-derived de-identified database. The primary outcome was first diagnosis of brain metastases. We estimated prevalence and incidence of brain metastases by MBC subtype, including HER2-low and therapy line. We used the cumulative incidence function to estimate brain metastases risk in patients without brain metastases at initiation of systemic therapy. All P-values are 2-sided, and a P-value ≤.05 indicates statistical significance. RESULTS:Among 18 075 patients with MBC, 1102 (6.1%) had at least 1 brain metastasis at first-line therapy initiation. For the remaining 16 973 patients, cumulative incidence of brain metastases at 60 months was 10% in patients with hormone receptor-positive (HR+)/HER2- disease, 23% for HR+/HER2+ disease, 34% for HR-/HER2+ disease, and 22% for triple-negative breast cancer (TNBC). HER2-low expression within HR+/HER2- and TNBC subtypes had no impact on brain metastases incidence. Brain metastases prevalence increased per line of therapy for patients with all breast cancer subtypes. CONCLUSIONS:Brain metastases incidence increases per line of therapy for every MBC subtype. The HER2-low biomarker does not impact brain metastases incidence within historical subtypes.

PURPOSE:Cyclin-dependent kinase 4/6 inhibitors can significantly extend survival when given in combination with endocrine therapy in patients with hormone receptor-positive metastatic breast cancer. However, their activity has been relatively underexplored in patients with metastatic triple-negative breast cancer (mTNBC). PATIENTS AND METHODS:We conducted a single-arm phase II study of abemaciclib monotherapy in patients with Rb-positive mTNBC. Patients were treated with abemaciclib 200 mg orally twice daily until disease progression, unacceptable toxicity, or withdrawal of consent. The primary endpoint was the objective response rate; secondary endpoints included progression-free survival (PFS), overall survival (OS), clinical benefit rate, disease control rate, and safety and tolerability. RESULTS:A total of 27 patients were enrolled before the trial was closed early because of slow accrual. Patients had received a median of two lines of systemic therapy in the metastatic setting prior to enrollment. After a median follow-up of 28.5 months, the objective response rate was 0%, the clinical benefit rate was 14.8%, and the disease control rate was 22.2%. The median PFS was 1.94 months (95% confidence interval, 1.84-11.47), and the median OS was 8.44 months (95% confidence interval, 4.57-15.57). Median PFS and OS did not differ significantly based on androgen receptor and PD-L1 status. Pretreatment gene expression profiling of tumor tissue provided some hypothesis-generating insights into biological features associated with clinical benefit in this study. The most common treatment-related adverse events of grade 2 or higher were diarrhea (40.7%), neutropenia (40.7%), anemia (29.6%), and nausea (29.6%). CONCLUSIONS:Abemaciclib monotherapy did not show clinical activity in patients with pretreated Rb-positive mTNBC.

Abstract Background: Cyclin-dependent kinase (CDK) 4/6 inhibitors can significantly extend survival when given in combination with endocrine therapy as treatment for hormone receptor-positive metastatic breast cancer. Preclinical studies suggest CDK4/6 inhibitor monotherapy might also be effective in a subset of triple-negative breast cancers (TNBCs), including those that express a functional retinoblastoma (RB) protein and/or those of the Luminal Androgen Receptor (LAR) subtype. Currently, the clinical activity of CDK4/6 inhibitor therapy in TNBC has not been reported. Methods: We conducted a single-arm phase II study of abemaciclib monotherapy in patients with locally advanced or metastatic TNBC. Key eligibility criteria included: (i) Measurable disease by RECIST 1.1; (ii) Between 1-3 prior lines of systemic therapy for advanced TNBC; (iii) RB-positive tumor (defined as ≥ 50% of tumor cells staining positive for RB by immunohistochemistry [archival or fresh sample] on central testing); (iv) ECOG PS 0/1. Patients were treated with abemaciclib 200 mg orally (28-day cycles) twice daily until disease progression, unacceptable toxicity, withdrawal of consent, or death. Tumor biopsies were mandatory at baseline and C2D1 if tumor tissue was safely accessible. The primary outcome was objective response rate (ORR). Key secondary endpoints included progression-free survival (PFS), overall survival (OS), clinical benefit rate (CBR: CR + PR + SD ≥ 24 weeks), and safety and tolerability. The study had a two-stage design. Thirteen patients were enrolled in the first stage, with the plan to enrol a further 25 patients if at least one objective response was observed. Results: One unconfirmed partial response was observed in stage 1, and a total of 27 patients were enrolled before the trial was closed early due to slow accrual. The median age was 61 years and patients had received a median of 2 prior lines of systemic therapy for metastatic disease. Twelve patients had received prior immunotherapy. After a median follow-up of 28.5 months, the confirmed ORR was 0% and the CBR was 15%, with 4 of 27 patients experiencing stable disease for ≥ 24 weeks. The median PFS was 1.94 months (95% CI: 1.8 – 11.5 months), and the median OS was 8.44 months (95% CI: 4.6 – 15.6 months). There was no significant difference in PFS or OS between patients with PD-L1-positive versus PD-L1-negative disease, or Androgen Receptor (AR) positive versus negative tumors by immunohistochemistry. The most common adverse events of grade 2 or higher were diarrhea (41%), neutropenia (41%), anemia (30%), and nausea (30%). RNA-sequencing of baseline biopsies has been performed to identify biomarkers associated with clinical benefit, and results will be presented at the meeting. Conclusions: Abemaciclib monotherapy did not show clinical activity in patients with Rb+ metastatic TNBC. This finding suggests that future trials of CDK4/6 inhibition as monotherapy in TNBC are not warranted. Citation Format: Shom Goel, Bojana Jovanović, Xiangying Chu, Melissa Hughes, Ayesha Mohammed-Abreu, Julie Kasparian, Timothy Erick, Molly DiLullo, Eileen Wrabel, Rinath Jeselsohn, Nabihah Tayob, Stuart Schnitt, Nancy Lin, Sara Tolaney. A phase 2 study of abemaciclib monotherapy for patients with retinoblastoma-positive (Rb+), triple-negative metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS12-06.

For patients with metastatic triple-negative breast cancer (TNBC), treatment with pembrolizumab is dependent on the accurate determination of programmed death ligand 1 (PD-L1) expression using immunohistochemistry (IHC). This study evaluated the interobserver concordance in assessing PD-L1 expression on TNBC samples using the commercial 22C3 IHC assay and an in-house assay based on the E1L3N antibody. Concordance between the 22C3 and the E1L3N IHC assays was evaluated on TNBC samples read by a commercial laboratory and a Brigham and Women’s Hospital breast pathologist (BWH reader). Each slide was given a PD-L1 combined positive score (CPS) and was considered PD-L1 positive or negative based on the CPS cutoff of 10. Interobserver concordance for the assays was also evaluated on a subset of samples between 2 and 3 independent readers. On 71 samples, 2 independent readers (1 BWH reader and commercial laboratory) using E1L3N and 22C3, respectively, reached agreement on PD-L1 status (positive/negative) on 64 samples (90.1%). Using 22C3, 2 independent readers reached agreement on PD-L1 status on 30 of 36 samples (83.3%), and 3 independent readers reached agreement on 16 of 27 samples (59.3%). Using E1L3N, 2 BWH readers reached agreement on PD-L1 status on 18 of 27 samples (66.7%). Three BWH readers reached an agreement on 2 of 12 of the most challenging samples (16.7%). In conclusion, concordance between E1L3N and 22C3 testing using CPS for PD-L1 in metastatic TNBC was >90%. However, certain cases were challenging to agree upon using current threshold criteria, highlighting the need for more standardized evidence-based methods to assess PD-L1 expression.

Cancer patients, due to their immunocompromised status, are at an increased risk for severe SARS-CoV-2 infection. Since severe SARS-CoV-2 infection causes multiple organ damage through IL-6-mediated inflammation while stimulating hypoxia, and malignancy promotes hypoxia-induced cellular metabolic alterations leading to cell death, we propose a mechanistic interplay between both conditions that results in an upregulation of IL-6 secretion resulting in enhanced cytokine production and systemic injury. Hypoxia mediated by both conditions results in cell necrosis, dysregulation of oxidative phosphorylation, and mitochondrial dysfunction. This produces free radicals and cytokines that result in systemic inflammatory injury. Hypoxia also catalyzes the breakdown of COX-1 and 2 resulting in bronchoconstriction and pulmonary edema, which further exacerbates tissue hypoxia. Given this disease model, therapeutic options are currently being studied against severe SARS-COV-2. In this study, we review several promising therapies against severe disease supported by clinical trial evidence—including Allocetra, monoclonal antibodies (Tixagevimab–Cilgavimab), peginterferon lambda, Baricitinib, Remdesivir, Sarilumab, Tocilizumab, Anakinra, Bevacizumab, exosomes, and mesenchymal stem cells. Due to the virus’s rapid adaptive evolution and diverse symptomatic manifestation, the use of combination therapies offers a promising approach to decrease systemic injury. By investing in such targeted interventions, cases of severe SARS-CoV-2 should decrease along with its associated long-term sequelae and thereby allow cancer patients to resume their treatments.

The antigenic targets of immunity and the role of vaccination in breast cancer are unknown. We performed a phase I study of an autologous GM-CSF-secreting breast cancer vaccine in patients with metastatic and stage II–III breast cancer. Tumor cells from patients with metastatic (n = 15) and stage II–III (n = 7) disease were transduced with a replication-defective adenoviral vector encoding GM-CSF, and then irradiated. Twelve and seven patients with metastatic and stage II–III disease, respectively, received weekly vaccination for three weeks, followed by every other week until disease progression or vaccine supply was exhausted (metastatic) or until six total vaccine doses were administered (stage II–III). Among those patients with metastatic disease who received vaccinations, eight had progressive disease at two months, three had stable disease for 4–13 months, and one has had no evidence of disease for 13 years. Of the patients with stage II–III disease, five died of metastatic disease between 1.16 and 8.49 years after the start of vaccinations (median 6.24 years) and two are alive as of September 2021. Toxicities included injection site reactions, fatigue, fever, upper respiratory symptoms, joint pain, nausea, and edema. Four of five evaluable patients with metastatic disease developed a skin reaction with immune cell infiltration after the fifth injection of unmodified, irradiated tumor cells. We conclude that tumor cells can be harvested from patients with metastatic or stage II–III breast cancer to prepare autologous GM-CSF-secreting vaccines that induce coordinated immune responses with limited toxicity. clinicaltrials.gov, NCT00317603 (April 25, 2006) and NCT00880464 (April 13, 2009).

Purpose We sought to understand the attitudes of individuals with abnormal breast imaging findings prompting a diagnostic breast biopsy toward donation of blood, excised tissue, or percutaneous biospecimens for research, and to understand medical oncologists' attitudes toward research biospecimen collection in this population. Methods We included individuals who presented to a single academic medical center for a clinically indicated, image-guided, percutaneous breast biopsy. We administered a survey prior to knowledge of biopsy results to assess willingness to consider, entirely for research purposes, donating blood or excess excised breast tissue, or having additional biospecimens (AB) obtained during a clinically indicated percutaneous biopsy. We also surveyed breast medical oncologists from National Cancer Institute-designated cancer centers to assess attitudes toward approaching patients for biospecimen research. Results Overall, 53/63 patients responded to the survey; 70% would consider donating blood, 85% would consider donating excess excised breast tissue, and 32% would consider having AB obtained during a clinically indicated biopsy. Main motivating factors for considering AB included contributing to scientific knowledge and return of study or biopsy results, whereas anxiety and the potential discomfort were the main dissuading factors. Among 191 medical oncologists, most were very comfortable (59.2%), or somewhat comfortable (32.5%) asking patients to have AB obtained during a clinically indicated breast biopsy. Medical oncologists reported hesitancy to refer a patient for AB due to potential pain/discomfort, and other procedure risks. Conclusions Only one-third of individuals with breast imaging findings would consider consenting to AB during a diagnostic biopsy, whereas most were open to donating blood or excess excised breast tissue. Most medical oncologists would be comfortable asking patients to have AB obtained during the biopsy. Understanding patients' and oncologists' baseline attitudes may inform the design and approach to breast biospecimen-based research.

Background The objective of this study was to describe the perspective of patients with early breast cancer toward research biopsies. The authors hypothesized that more patients at academic sites than at community-based sites would be willing to consider these procedures. Methods In total, 198 patients with early stage breast cancer were recruited from 3 academic centers (n = 102) and from 1 community oncology practice (n = 96). The primary objective was to compare the proportion of patients willing to consider donating excess tissue biospecimens from surgery, from a clinically indicated breast biopsy, or from a research purposes-only biopsy (RPOB) between practice types. Results Most patients (93% at academic sites, 94% at the community oncology site) said they would consider donating excess tissue from surgery for research. One-half of patients from academic or community sites would consider donating tissue from a clinically indicated breast biopsy. On univariate analysis, significantly fewer patients from academic sites would consider an RPOB (22% at academic sites, 42% at the community site; P = .003); however, this difference was no longer significant on multivariate analysis (P = .96). Longer transportation times and unfavorable prior experiences were associated with less willingness to consider an RPOB on multivariate analysis. Significantly fewer patients from academic sites (14%) than from the community site (35%) would consider a research biopsy in a clinical trial (P = .04). Contributing to scientific knowledge, return of results, and a personal request by their physician were the strongest factors influencing patients' willingness to undergo research biopsies. Conclusions The current results rejected the hypothesis that more patients with early breast cancer at academic sites would be willing to donate tissue biospecimens for research compared with those at community oncology sites. These findings identify modifiable factors to consider in biobanking studies and clinical trials.

3135 Background: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel betacoronavirus that causes the respiratory illness coronavirus disease 2019 (COVID-19). COVID-19 ranges in severity from an asymptomatic viral infection to life-threatening cases of pneumonia, acute respiratory distress syndrome (ARDS), multi-organ damage and sepsis. Cancer patients are at an increased risk of severe SARS-CoV-2 infection due to their immunocompromised status. We propose a mechanism by which SARS-CoV-2 infection causes multiple organ damage through IL-6-mediated inflammation and hypoxia-induced cellular metabolic alterations leading to cell death. Hypoxia is also induced by malignancy due to alterations in metabolism, resulting in greater IL-6 secretion. Methods: To highlight the possible effect of active cancer on the likelihood of hypoxia in COVID-19, we analyzed the correlation between cancer status and the severity of COVID-19 from the COVID-19 and Cancer Consortium data registry. For cancer status, we looked at progressive cancer and remission of cancer only -- those being the two extremes of presence and absence of uncontrolled cancer. Similar to prior studies, the severity of COVID-19 was used as an indication of hypoxia. Results: We observed a 24% positive deviation between expected and actual number of patients with actively progressing cancer who had hypoxic COVID-19 (moderate to severe), and a 26.9% negative deviation between expected and actual number of patients with active cancer who had no hypoxia with COVID-19 (p<0.0001). Conversely, for patients with cancer in remission, there was only a +5.8% and -5.1% deviation between expected and actual number of patients who did not have hypoxia and who had hypoxia, respectively. Conclusions: These results suggest that in the presence of poorly controlled malignancy, there is an increased likelihood of hypoxia in patients with COVID-19, thereby exacerbating downstream cytokine release syndrome and contributing to prolonged systemic inflammatory injury. Appreciating this pathway, future therapies can be developed to target the pathogenesis of both diseases and prevent progression, as seen with mesenchymal stem cells, which demonstrated a 91% overall survival and 100% survival in patients younger than 85 years old at one month after a single treatment.[Table: see text]

Rationale: Transformed MUC1 (tMUC1) is a cancer-associated antigen that is overexpressed in >90% of triple-negative breast cancers (TNBC), a highly metastatic and aggressive subtype of breast cancer.TAB004, a murine antibody targeting tMUC1, has shown efficacy for the targeted delivery of therapeutics to cancer cells.Our aim was to evaluate humanized TAB004 (hTAB004) as a potential theranostic for TNBC.Methods: The internalization of hTAB004 in tMUC1 expressing HCC70 cells was assessed via fluorescent microscopy.hTAB004 was DOTA-conjugated and radiolabeled with Indium-111 or Actinium-225 and tested for stability and tMUC1 binding (ELISA, flow cytometry).Lastly, in vivo biodistribution (SPECT-CT), dosimetry, and efficacy of hTAB004 were evaluated using a TNBC orthotopic mouse model.Results: hTAB004 was shown to bind and internalize into tMUC1-expressing cells.A production method of 225 Ac-DOTA-hTAB004 (yield>97%, RCP>97% SA=5 kBq/µg) and 111 In-DOTA-hTAB004 (yield>70%, RCP>99%, SA=884 kBq/µg) was developed.The labeled molecules retained their affinity to tMUC1 and were stable in formulation and mouse serum.In NSG female mice bearing orthotopic HCC70 xenografts, the in vivo tumor concentration of 111 In-DOTA-hTAB004 was 65 ± 15 %ID/g (120 h post injection).A single 225 Ac-DOTA-hTAB004 dose (18.5 kBq) caused a significant reduction in tumor volume (P<0.001,day 22) and increased survival compared to controls (P<0.007).The human dosimetry results were comparable to other clinically used agents. Conclusion:The results obtained with hTAB004 suggest that the 111 In/ 225 Ac-DOTA-hTAB004 combination has significant potential as a theranostic strategy in TNBC and merits further development toward clinical translation.

The world is currently embroiled in a pandemic of coronavirus disease 2019 (COVID-19), a respiratory illness caused by the novel betacoronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The severity of COVID-19 disease ranges from asymptomatic to fatal acute respiratory distress syndrome. In few patients, the disease undergoes phenotypic differentiation between 7 and 14 days of acute illness, either resulting in full recovery or symptom escalation. However, the mechanism of such variation is not clear, but the facts suggest that patient's immune status, comorbidities, and the systemic effects of the viral infection (potentially depending on the SARS-CoV-2 strain involved) play a key role. Subsequently, patients with the most severe symptoms tend to have poor outcomes, manifest severe hypoxia, and possess elevated levels of pro-inflammatory cytokines (including IL-1β, IL-6, IFN-γ, and TNF-α) along with elevated levels of the anti-inflammatory cytokine IL-10, marked lymphopenia, and elevated neutrophil-to-lymphocyte ratios. Based on the available evidence, we propose a mechanism wherein SARS-CoV-2 infection induces direct organ damage while also fueling an IL-6-mediated cytokine release syndrome (CRS) and hypoxia, resulting in escalating systemic inflammation, multi-organ damage, and end-organ failure. Elevated IL-6 and hypoxia together predisposes patients to pulmonary hypertension, and the presence of asymptomatic hypoxia in COVID-19 further compounds this problem. Due to the similar downstream mediators, we discuss the potential synergistic effects and systemic ramifications of SARS-CoV-2 and influenza virus during co-infection, a phenomenon we have termed "COVI-Flu." Additionally, the differences between CRS and cytokine storm are highlighted. Finally, novel management approaches, clinical trials, and therapeutic strategies toward both SARS-CoV-2 and COVI-Flu infection are discussed, highlighting host response optimization and systemic inflammation reduction.

The liver harbors two main innate lymphoid cell (ILC) populations: conventional NK (cNK) cells and tissue-resident NK (trNK) cells. Using the MCMV model of infection, we find that, in contrast to liver cNK cells, trNK cells initially undergo a contraction phase followed by a recovery phase to homeostatic levels. The contraction is MCMV independent because a similar phenotype is observed following poly(I:C)/CpG or α-GalCer injection. The rapid contraction phase is due to apoptosis, whereas the recovery phase occurs via proliferation in situ. Interestingly, trNK cell apoptosis is not mediated by fratricide and not induced by liver lymphocytes or inflammatory cytokines. Instead, we find that trNK cell apoptosis is the consequence of an increased sensitivity to lactic acid. Mechanistic analysis indicates that trNK cell sensitivity to lactate is linked to impaired mitochondrial function. These findings underscore the distinctive properties of the liver-resident NK cell compartment.

Rationale: Transformed MUC1 (tMUC1) is a cancer-associated antigen that is overexpressed in >90% of triple-negative breast cancers (TNBC), a highly metastatic and aggressive subtype of breast cancer. TAB004, a murine antibody targeting tMUC1, has shown efficacy for the targeted delivery of therapeutics to cancer cells. Our aim was to evaluate humanized TAB004 (hTAB004) as a potential theranostic for

Immunotherapy is a promising approach to treating cancer. Mucin1 (MUC1), an epithelial glycoprotein, is hypo-glycosylated and overexpressed on epithelial cancers. This renders it a promising target for potential immunotherapeutic approaches. However, MUC1 has also been identified on T cells, which might complicate its potential as a target for immunotherapies.

In recent years, vaccines against tumor antigens have shown potential for combating invasive cancers, including primary tumors and metastatic lesions. This is particularly pertinent for breast cancer, which is the second-leading cause of cancer-related death in women. MUC1 is a glycoprotein that is normally expressed on glandular epithelium, but is overexpressed and under-glycosylated in most human cancers, including the majority of breast cancers. This under-glycosylation exposes the MUC1 protein core on the tumor-associated form of the protein. We have previously shown that a vaccine consisting of MUC1 core peptides stimulates a tumor-specific immune response. However, this immune response is dampened by the immunosuppressive microenvironment within breast tumors. Thus, in the present study, we investigated the effectiveness of MUC1 vaccination in combination with four different drugs that inhibit different components of the COX pathway: indomethacin (COX-1 and COX-2 inhibitor), celecoxib (COX-2 inhibitor), 1-methyl tryptophan (indoleamine 2,3 dioxygenase inhibitor), and AH6809 (prostaglandin E2 receptor antagonist). These treatment regimens were explored for the treatment of orthotopic MUC1-expressing breast tumors in mice transgenic for human MUC1. We found that the combination of vaccine and indomethacin resulted in a significant reduction in tumor burden. Indomethacin did not increase tumor-specific immune responses over vaccine alone, but rather appeared to reduce the proliferation and increase apoptosis of tumor cells, thus rendering them susceptible to immune cell killing.

The presence of tissue-resident NK (trNK) cells in the liver has only recently been appreciated. While the role of conventional NK cells (cNK) has been well-characterized, the trNK cell function is only beginning to be elucidated. Using the MCMV model of infection, we found that in contrast to liver cNK cells, trNK cells initially undergo a contraction phase followed by a recovery phase to homeostatic levels. The contraction phase is independent of the virus itself as a similar phenotype is observed following Poly I:C/CpG or α-GalCer injection. Interestingly, this initial contraction phase is independent of other liver lymphocyte subsets as it occurs even in infected lymphopenic animals, suggesting it is provoked by the inflammatory milieu. We also found that the contraction phase is partially due to cell apoptosis while the recovery phase occurs via proliferation in situ. These findings underscore the unique homeostatic properties and kinetics of liver resident NK cell compartment.

The murine lacrimal gland (LG), which produces crucial components of the ocular tear film, contains a population of NK cells. LG NK cells appear to belong to the conventional NK cell lineage, based on their cell surface receptor and transcription factor expression, absence in NFIL3−/− mice, and lack of RORγt expression during development. LG NK cells produce IFN-γ during the early stages of systemic murine CMV (MCMV) infection. This effector response occurs in the absence of noticeable MCMV replication in the LG, indicating that LG NK cells are being activated by soluble factors. However, the magnitude of LG NK cell IFN-γ production during MCMV infection is significantly lower than for spleen and liver NK cells. Adoptive-transfer experiments in lymphopenic mice revealed that this hyporesponsive phenotype is tissue specific, which indicates that LG NK cells can produce a robust effector response.