The Role of Progesterone in Elf5 Activation and Milk Component Synthesis for Cell-Cultured Milk Production in MAC-T Cells

Prolactin is essential for mammary gland development and lactation. Progesterone also induces ductal branching and alveolar formation via initial secretory differentiation within the mammary gland. Herein, we aimed to evaluate the role of progesterone as a prolactin substitute for the production of cell-cultured milk components in MAC-T cells. Cells were treated with various hormones such as prolactin (PRL), progesterone (P4), 17β-estradiol (E2), cortisol (COR), and insulin (INS) for 5 d. MAC-T cells cultured in a P4 differentiation media (2500 ng/mL of P4, 25 ng/mL of E2, 25 ng/mL of COR, and 25 ng/mL of INS) showed similar levels of E74-like factor 5 (Elf5) and milk component synthesis (α-casein, β-casein, α-lactalbumin, β-lactoglobulin, and triglycerides) compared to those cultured in a PRL differentiation media (5000 ng/mL of PRL, 500 ng/mL of CORT, and 50 ng/mL of INS). The levels of α-casein and triglycerides in the optimal P4 differentiation media were present at comparable levels to those in the PRL differentiation media. Our results demonstrated that P4 induces the activation of Elf5 and the synthesis of milk components in MAC-T cells, similar to PRL. Therefore, P4 may be used as an effective substitute of PRL for cell-cultured milk production in in vitro frameworks.

Choline regulation of triglycerides synthesis through ubiquintination pathway in MAC-T cells

This study aims to investigate the regulatory mechanism of choline (CH) on triglyceride (TG) synthesis in cows, with a specific focus on its potential association with high milk fat percentage in the gut of the Zhongdian yak. By employing combined metagenomics and metabolomics analysis, we establish a correlation between CH and milk fat production in yaks. Bovine mammary epithelial cells (MAC-T) were exposed to varying CH concentrations, and after 24 h, we analyzed the expression levels of key proteins (membrane glycoprotein CD36 (CD36); adipose differentiation-related protein (ADFP); and ubiquintin (UB)), cellular TG content, lipid droplets, and cell vitality. Additionally, we evaluated the genes potentially related to the CH-mediated regulation of TG synthesis using real-time qPCR. CH at 200 μM significantly up-regulated CD36, ADFP, UB, and TG content. Pathway analysis reveals the involvement of the ubiquitination pathway in CH-mediated regulation of TG synthesis. These findings shed light on the role of CH in controlling TG synthesis in MAC-T cells and suggest its potential as a feed additive for cattle, offering possibilities to enhance milk fat production efficiency and economic outcomes in the dairy industry.

Effects of Serotonin on Cell Viability, Permeability of Bovine Mammary Gland Epithelial Cells and Their Transcriptome Analysis

Serotonin (5-HT) has been reported to play an important role in mammary gland involution that is defined as the process through which the gland returns to a nonlactating state. However, the overall picture of the regulatory mechanisms of 5-HT and the effects of serotonylation on mammary gland involution still need to be further investigated. The current study aimed to investigate the effects of 5-HT on global gene expression profiles of bovine mammary epithelial cells (MAC-T) and to preliminarily examine whether the serotonylation involved in the mammary gland involution by using Monodansylcadaverine (MDC), a competitive inhibitor of transglutaminase 2. Results showed that a high concentration of 5-HT decreased viability and transepithelial electrical resistance (TEER) in MAC-T cells. Transcriptome analysis indicated that 2477 genes were differentially expressed in MAC-T cells treated with 200 μg/mL of 5-HT compared with the control group, and the Notch, p53, and PI3K-Akt signaling pathways were enriched. MDC influenced 5-HT-induced MAC-T cell death, fatty acid synthesis, and the formation and disruption of tight junctions. Overall, a high concentration of 5-HT is able to accelerate mammary gland involution, which may be regulated through the Notch, p53, and PI3K-Akt signaling pathways. Serotonylation is involved in bovine mammary gland involution.

The anti-apoptotic and anti-autophagic effects of EPO through PI3K/Akt/mTOR signaling pathway in MAC-T cells

Lipopolysaccharide (LPS) is an important inflammatory and infected factor of bacterial mastitis, which treated bovine mammary epithelial cells (MAC-T) in our previous studies, as mastitis cells model in vitro. Erythropoietin (EPO) is a well-known hematopoietic hormone with antioxidative, anti-apoptotic, and anti-inflammatory roles. We hypothesized that EPO might regulate the apoptosis and autophagy to attenuate the inflammation of mastitis. Western blot, RT-PCR, transmission electron microscope analysis and Annexin V-FITC/PI were used to evaluate the regulation of EPO on apoptosis and autophagy in inflammatory MAC-T cells. These results demonstrated that EPO promoted the proliferation of MAC-T cells. Meanwhile, EPO had a better anti-inflammatory effect in MAC-T cells with LPS treatment. Certainly, EPO also showed anti-apoptotic and anti-autophagic effects. Interestingly, we found that the beneficial effect of EPO on inflammatory MAC-T cells depended on the PI3K/Akt/mTOR signaling pathway, which was involved in the regulation of apoptosis and autophagy. Generally, this study provides an insight for EPO to inhibit apoptosis and autophagy of inflammatory MAC-T cells via PI3K/Akt/mTOR signaling pathway.

Deoxynivalenol exposure inhibits biosynthesis of milk fat and protein by impairing tight junction in bovine mammary epithelial cells

Deoxynivalenol (DON) is one of the most common feed contaminants, and it poses a serious threat to the health of dairy cows. The existing studies of biological toxicity of DON mainly focus on the proliferation, oxidative stress, and inflammation in bovine mammary epithelial cells, while its toxicity on the biosynthesis of milk components has not been well documented. Hence, we investigated the toxic effects and the underlying mechanism of DON on the bovine mammary alveolar cells (MAC-T). Our results showed that exposure to various concentrations of DON significantly inhibited cell proliferation, induced apoptosis, and altered the cell morphology which was manifested by cell distortion and shrinkage. Moreover, the transepithelial electrical resistance (TEER) values of MAC-T cells exposed to DON were gradually decreased in a time- and concentration- dependent manner, but lactate dehydrogenase (LDH) leakage was significantly increased with the maximum increase of 2.4-fold, indicating the cell membrane and tight junctions were damaged by DON. Importantly, DON significantly reduced the synthesis of β-casein and lipid droplets, along with the significantly decreases of phospho-mTOR, phospho-4EBP1, phospho-JAK2, and phospho-STAT5. Gene expression profiles showed that the expressions of several genes related to lipid synthesis and metabolism were changed, including acyl-CoA synthetase short-chain family member 2 (ACSS2), fatty acid binding protein 3 (FABP3), 3-hydroxy-3-methylglutaryl-CoA synthase 1 (HMGCS1), and insulin-induced gene 1 (INSIG1). GO and KEGG enrichment analyses revealed that the differentially expressed genes (DEGs) were significantly enriched in ribosome, glutathione metabolism, and lipid biosynthetic process, which play important roles in the toxicological process induced by DON. Taken together, DON affects the proliferation and functional differentiation of MAC-T cells, which might be related to the cell junction disruption and morphological alteration. Our data provide new insights into functional differentiation and transcriptomic alterations of MAC-T cells after DON exposure, which contributes to a comprehensive understanding of DON-induced toxicity mechanism.

Choline attenuates heat stress-induced oxidative injury and apoptosis in bovine mammary epithelial cells by modulating PERK/Nrf-2 signaling pathway

Heat stress-induced decline in milk production and mammary glands dysfunction are economically important challenges facing the dairy industry, especially in summer. Choline is an organic water-soluble compound that can regulate a series of vital biological process, including cellular structural integrity and oxidative stress. However, it is unclear whether choline plays an anti-apoptosis and antioxidant effect in heat stress-induced mammary epithelial cells. This study aimed to determine the antioxidant effect of choline on heat stress-induced apoptosis and oxidative stress and its underlying molecular mechanism in bovine mammary epithelial cells (MAC-T cells). The MAC-T cells were divided into four treatment groups: control (37℃), choline (37℃), heat stress (HS, 42℃), and HS + choline. The results showed that heat stress up-regulated the HSP70 and HSP90 expression both in mRNA and protein, enhanced ROS accumulation, increased malondialdehyde (MDA) content, reduced the superoxide dismutase (SOD) and glutathione peroxidase (GSH-PX) activity, significantly increased the expression of caspase-3 and upregulated the ratio of Bax/Bcl-2 and ultimately lead to oxidative stress and apoptosis in MAC-T cells. However, choline pretreatment reversed the above phenomenon compared with the HS group. The HS + choline group inhibited heat stress-induced phosphorylation of PERK, nuclear translocation of Nrf-2 and the protein expression of GRP78. In addition, the ratio of Bax/Bcl-2 and the expression of caspase-3 were significantly reduced in HS + choline group, thereby reduced the HS-induced oxidative stress and apoptosis in MAC-T cells. In conclusion, choline attenuates heat stress-induced oxidative stress and apoptosis of MAC-T cells by modulating PERK/Nrf-2 pathway.

Curcumin Alleviates LPS-Induced Oxidative Stress, Inflammation and Apoptosis in Bovine Mammary Epithelial Cells via the NFE2L2 Signaling Pathway

Lipopolysaccharide (LPS) is an endotoxin, which may cause immune response and inflammation of bovine mammary glands. Mastitis impairs animal health and results in economic loss. Curcumin (CUR) is a naturally occurring diketone compound, which has attracted widespread attention as a potential anti-inflammatory antioxidant. The purpose of this study is to investigate whether CUR can reduce the damage of bovine mammary epithelial cells (MAC-T) induced by LPS and its underlying molecular mechanism. The MAC-T cell line was treated with different concentrations of LPS and CUR for 24 h. The results showed that CUR rescued the decrease of MAC-T cell viability and cell damage induced by LPS. At the same time, 10 µM CUR and 100 µg/mL LPS were used to treat the cells in the follow-up study. The results showed CUR treatment reduced the accumulation of reactive oxygen species (ROS), the expression of inflammatory cytokines (tumor necrosis factor-a (TNF-α), interleukin-8 (IL-8), IL-6 and IL-1β) and the rate of apoptosis induced by LPS. These effects were associated with the activation of the nuclear factor E2-related factor 2 (NFE2L2)-antioxidant response element (ARE) pathway coupled with inactivation of the nuclear factor-κB (NF-κB) inflammatory and caspase/Bcl2 apoptotic pathways.

Inhibition of PI3K/Akt/mTOR pathway by ammonium chloride induced apoptosis and autophagy in MAC-T cell

Ammonia is a harmful gas with a pungent odor, participates in the regulation of a variety of apoptosis and autophagy, which in turn affects the growth and differentiation of cells. To test the regulation of NH₃ on the apoptosis and autophagy of mammary epithelial cells, we selected NH₄Cl as NH₃ donor in vitro model. MTT and CCK-8 assay kits were employed to detect cell activity. Real-time quantitative PCR and western blot methods were used to detect the abundance of inflammatory molecules, apoptosis markers, and autophagy genes. We selected TUNEL kit and the Annexin-FITC/PI method to detect apoptosis. TEM analysis was used to detect autophagic vesicles, and MDC stain evaluated the formation of autophagosome. The results indicated that NH₄Cl reduced cell viability in a concentration-dependent manner and promoted cell inflammatory response, apoptosis, and autophagy. NH₄Cl stimulation notable increased the autophagosomes number. Interestingly, we also detected that the addition of LY294002 and Rapamycin inhibited the PI3K/Akt pathway and the mTOR pathway, respectively, resulting in changes in both apoptosis and autophagy. Therefore, we draw a conclusion that NH₃ may regulate the apoptosis and autophagic response of bovine mammary epithelial cells through the PI3K/Akt/mTOR signaling pathway. Further investigations on ammonia's function in other physiological respects, will be critical to provide theoretical help for the improvement of production performance. It will be also helpful for controlling the harmful gas ammonia concentration in the livestock house to protect the health of dairy cows.

Cytoprotective Effects of Taurine on Heat-Induced Bovine Mammary Epithelial Cells In Vitro

It is a widely known that heat stress induces a reduction in milk production in cows and impairs their overall health. Studies have shown that taurine protects tissues and organs under heat stress. However, there have yet to be studies showing the functions of taurine in mammary alveolar cells-large T antigen (MAC-T) (a bovine mammary epithelial cell line) cells under heat shock. Therefore, different concentrations of taurine (10 mM, 50 mM, and 100 mM) were tested to determine the effects on heat-induced MAC-T cells. The results showed that taurine protected the cells against heat-induced damage as shown by morphological observations in conjunction with suppressed the translocation and expression of heat shock factor 1 (HSF1). Moreover, taurine not only reversed the decline in antioxidase (superoxide dismutase (SOD) and glutathione peroxidase (GSH-PX)) activities but also attenuated the accumulation of malondialdehyde (MDA). Meanwhile, mitochondrial damage (morphology and complex I activity) resulting from heat exposure was mitigated. Taurine also alleviated the rates of cell apoptosis and markedly depressed the mRNA expressions of BCL2 associated X, apoptosis regulator (BAX) and caspase3. Furthermore, compared with the heat stress (HS) group, the protein levels of caspase3 and cleaved caspase3 were decreased in all taurine groups. In summary, taurine improves the antioxidant and anti-apoptosis ability of MAC-T cells thereby alleviates damage of cells due to heat insults.

Upregulation Sestrin2 protects against hydrogen peroxide-induced oxidative damage bovine mammary epithelial cells via a Keap1-Nrf2/ARE pathway

Sestrin2 (SESN2) is a highly conservative oxidative stress protein that can regulate energy metabolism, cell proliferation, apoptosis, and mitochondria autophagy processes. It plays a role as an antioxidant in various diseases. The aims of the present study were to explore the underlying role of SESN2 after hydrogen peroxide (H₂ O₂ ) treatment in bovine mammary epithelial cells (MAC-T cells) by the methods of knockout or overexpression of SESN2. The results show that knockout of Sestrin2 exacerbate apoptosis, upregulate the expressions of Bax/Bcl2 in H₂ O₂ -treated MAC-T cells. Moreover, knockout of SESN2 also promoted reactive oxygen species (ROS) generation and exacerbated oxidative damage in H₂ O₂ -treated MAC-T cells. On the contrary, overexpression of SESN2 decreased apoptosis by downregulation of Bax/Bcl2 level decreased ROS generation and blocked oxidative damage in H₂ O₂ -treated MAC-T cells. In addition, results indicate that the Kelch-like ECH-associated protein-1 (Keap1)-nuclear factor (erythroid-derived 2) like2 (Nrf2)/antioxidant response element (ARE) signaling pathway was activated by H₂ O₂ ; upregulation of SESN2 could relieve oxidative stress by inducing the expression of Keap1, Nrf2, HO-1, and NDPH: quinone oxidoreductase-1 protein. In conclusion, this study demonstrates that expression of SESN2 was significantly increased after H₂ O₂ treatment and that SESN2 can alleviate oxidative stress and cell apoptosis in H₂ O₂ -treated MAC-T cells through activation of the Keap1-Nrf2/ARE pathway.

H2S mediates apoptosis in response to inflammation through PI3K/Akt/NFκB signaling pathway

OBJECTIVES

Hydrogen sulfide (H₂S) is involved in regulating cell apoptosis and proliferation. However, The effects and mechanism of H₂S on the apoptosis of mammary epithelial cells that suffer from an inflammatory response remain unknown.

RESULTS

An inflammatory cell model was used to explore whether exogenous H₂S regulates lipopolysaccharides (LPS)-induced cell proliferation and apoptosis. We found that H₂S affected cell viability, the inflammatory response and apoptosis in LPS-treated cells in a concentration-dependent manner. Moreover, exogenous H₂S rescued LPS-induced cystathionine γ-lyase (CSE) inhibition and cystathionine β-synthase (CBS) synthesis. Interestingly, in cells undergoing inflammation-induced apoptosis, H₂S activated the PI3K/Akt and NFκB signal pathways both tested concentrations. Akt appeared to be a key crosstalk molecule that played a "bridge" role.

CONCLUSIONS

H₂S regulates LPS-induced inflammation and apoptosis by activating the PI3K/Akt/NFκB signaling pathway. Hence, NaHS may be clinically useful for preventing or treating mastitis.

l-Lactate Dehydrogenase B Chain Associated with Milk Protein Content in Dairy Cows

This study aimed to explore genes associated with milk protein content in dairy cows and their relationships with l-leucine. Ten primiparous Holstein cows (93.8 ± 11.56 milking days) fed the same diet were divided into two groups depending on their milk protein contents (group High, 3.34 ± 0.10%; and group Low, 2.86 ± 0.05%). Milk epithelial cells (MECs) were isolated from the collected morning milk and differentially expressed proteins in MECs were explored by two-dimensional gel electrophoresis (2-DE). Then, the mRNA expression of these proteins was detected by real time PCR in MAC-T cells incubated with three different media named positive control (PC), negative control (NC), and l-leucine depletion (NO-leu). Results showed that ten proteins were differentially expressed in MECs from cows in group High. They included seven down-regulated ones (heat shock protein beta-1 (HSPB1), 78 kDa glucose-regulated protein (GRP-78), l-lactate dehydrogenase B chain (LDH-B), malate dehydrogenase, cytoplasmic (MDH1), annexin I (ANXA1), cytokeratin-7 (CK-7), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH)), and three up-regulated ones (prohibitin (PHB), beta casein (CSN2), and alpha S1 casein (CSN1S1)). When l-leucine was depleted from the medium, not only proteins content was lowered (p < 0.05), but also the LDH-B mRNA expression was decreased in MAC-T cells (p < 0.05). In conclusion, LDH-B is negatively associated with the milk protein content of dairy cows and has a positive association with l-leucine.

Corrigendum: Staphylococcus aureus Bacteriophage Suppresses LPS-Induced Inflammation in MAC-T Bovine Mammary Epithelial Cells

[This corrects the article DOI: 10.3389/fmicb.2018.01614.].

Staphylococcus aureus Bacteriophage Suppresses LPS-Induced Inflammation in MAC-T Bovine Mammary Epithelial Cells

Several previous studies have shown that bacteriophages can significantly affect the production of various cytokines. The aim of this present study was to investigate the inflammatory effects and mechanisms of bacteriophage vB_SauM_JS25 in stimulated MAC-T bovine mammary epithelial cells by real-time polymerase chain reaction (PCR) and Western blotting. Experiments show that vB_SauM_JS25 reduces Staphylococcus aureus- or lipopolysaccharide (LPS)-induced levels of tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6, IL-8, IL-10, and regulated on activation, normal T cell expressed and secreted (RANTES) mRNA in MAC-T cells, in a manner expected to be unrelated to its antibacterial action. Moreover, S. aureus bacteriophage vB_SauM_JS25 suppressed the LPS-induced phosphorylation of nuclear factor (NF)-κB p65, which may represent an important mechanism mediating these effects. A carefully regulated balance between activation and inhibition by bacteriophages must be kept avoiding inappropriate inflammatory responses. The ability of vB_SauM_JS25 to influence the immune response highlights the potential development and application of bacteriophage-based therapies and may represent a novel anti-inflammatory therapeutic strategy.

Oxidative stress-induced inflammatory responses and effects of N-acetylcysteine in bovine mammary alveolar cells. J DAIRY RES. 2017;84:418-425. [PMID: 29154739 DOI: 10.1017/s002202991700067x]

Bovine mastitis, an inflammation of the udder, results in reduced milk production and poor milk quality. Mastitis is usually, but not always, a response to pathogen infection. High milk yield can produce oxidative stress in the mammary tissue. High milk yield is also known to be associated with bovine mastitis. Thus, in the current study, we hypothesised that oxidative stress increases inflammatory responses in bovine mammary cells. To examine the hypothesis, we produced cellular oxidative stress and investigated resulting inflammatory responses in bovine mammary alveolar cells (MAC-T). To produce oxidative stress, cells were treated with the reactive oxygen species (ROS; e.g., superoxide anion)-producing agent, menadione (MD; 0-10 µm; 6 h). To ensure the ROS-induced responses, cells were pretreated with an antioxidant NAC (0-10 mm; 1 h). Results showed that MD elevated intracellular ROS levels and protein expression of cyclooxygenase-2 (COX-2), a biomarker of inflammation. Pretreatment of cells with NAC attenuated MD-induced COX-2 expression by scavenging intracellular ROS and enhancing intracellular glutathione levels. MD-induced COX-2 expression was mediated by activation of extracellular signal receptor-activated kinase 1/2 (ERK1/2), Akt, and nuclear factor-kappa B (NF-κB). NAC attenuated activation of these intracellular signalling molecules. Treatment of cells with pharmacological inhibitors for ERK1/2, Akt, and NF-κB confirmed the association of these signalling pathways in MD-induced COX-2 expression. These results support our hypothesis that oxidative stress, which is found in high-yielding dairy cows, can produce cellular inflammation in bovine mammary alveolar cells and prevention of oxidative stress can attenuate such pathological responses. This may be relevant for cases of clinical mastitis for which no pathogen can be isolated.

Effects of vitamin D and its metabolites on cell viability and Staphylococcus aureus invasion into bovine mammary epithelial cells

Vitamin D has been found have various biological effects that may be potent in preventing bovine mastitis. Two forms of vitamin D, vitamin D₂ (D₂) and vitamin D₃ (D₃), can be hydroxylated to functional metabolites in cattle. The objectives of the present study were to investigate the potential of vitamin D compounds for controlling bovine mastitis using in vitro cell models, and to compare the differences between D₂ and D₃ compounds. Results showed that D₂ compounds have comparable effects to their D₃ analogues on inhibiting MAC-T cell viability in vitro. S. aureus growth was inhibited by high concentrations of D₂, D₃, 25(OH)D₂ and 25(OH)D₃. 25(OH)D₂ and 25(OH)D₃ induced CYP24A1 expression but reduced VDR mRNA expression, whereas the expression of CYP27B1, occludin, and E-cadherin did not change. Additionally, the induction of CYP24A1 expression by 25(OH)D₃ was higher than that of 25(OH)D₂, which may contribute to their differences in inhibiting cell viability. S. aureus invaded into MAC-T cells and universally inhibited gene expressions. Pre-treat MAC-T cells with 25(OH)D₂ reduced S. aureus adhesion while pre-treatment with 25(OH)D₃ inhibited S. aureus invasion, but neither of the compounds attenuated the S. aureus-induced gene expression reduction. In conclusion, the present study shows that D₂ compounds have comparable effects on inhibiting cell viability and S. aureus invasion to their D₃ analogues in vitro, suggesting that D₂ and its metabolites have potential in controlling bovine mastitis.

Intracellular Staphylococcus aureus Control by Virulent Bacteriophages within MAC-T Bovine Mammary Epithelial Cells

Bacteriophages (phages) are known to effectively kill extracellular multiplying bacteria. The present study demonstrated that phages penetrated bovine mammary epithelial cells and cleared intracellular Staphylococcus aureus in a time-dependent manner. In particular, phage vB_SauM_JS25 reached the nucleus within 3 h postincubation. The phages had an endocytotic efficiency of 12%. This ability to kill intracellular host bacteria suggests the utility of phage-based therapies and may protect patients from recurrent infection and treatment failure.

MAC-T cells as a tool to evaluate lentiviral vector construction targeting recombinant protein expression in milk

Prior to generating transgenic animals for bioreactors, it is important to evaluate the vector constructed to avoid poor protein expression. Mammary epithelial cells cultured in vitro have been proposed as a model to reproduce the biology of the mammary gland. In the present work, three lentiviral vectors were constructed for the human growth hormone (GH), interleukin 2 (IL2), and granulocyte colony-stimulating factor 3 (CSF3) genes driven by the bovine β-casein promoter. The lentiviruses were used to transduce mammary epithelial cells (MAC-T), and the transformed cells were cultured on polystyrene in culture medium with and without prolactin. The gene expression of transgenes was evaluated by PCR using cDNA, and recombinant protein expression was evaluated by Western-blotting using concentrated medium and cellular extracts. The gene expression, of the three introduced genes, was detected in both induced and non induced MAC-T cells. The human GH protein was detected in the concentrated medium, whereas CSF3 was detected in the cellular extract. Apparently, the cellular extract is more appropriate than the concentrated medium to detect recombinant protein, principally because concentrated medium has a high concentration of bovine serum albumin. The results suggest that MAC-T cells may be a good system to evaluate vector construction targeting recombinant protein expression in milk.

Virulence of Culture Filtrate from Heat-Injured and Repaired Listeria Strains: Assay on Bovine Mammary Epithelial (MAC-T) Cells

The cytotoxic effect of culture filtrates from healthy, heat-stressed, and repaired Listeria monocytogenes and L. innocua on the bovine mammary epithelial cell line MAC-T was examined. Culture filtrates were collected from Listeria spp. following treatments which included: (i) 18 h of growth of Listeria at 37°C; (ii) sublethal heat treatment at 56°C for 50 minutes; (iii) repair of the injured cells at 37°C for 7 h; (iv) growth of repaired bacterial cells at 37°C for 36 h; and (v) heat injury at 56°C for 50 min of the cell population obtained after the initial repair and growth. Strains chosen for study included two genetic mutants of L. monocytogenes : a hemolysin-negative mutant, CNL 85/162 (Hly^-) and a hemolysin-positive revertant, CNL 85/163 (Hly⁺). Culture filtrates obtained from Hly^- bacteria did not prevent adhesion of the mammary epithelial cells and slightly stimulated their growth. In contrast, culture filtrates from Hly⁺ bacteria grown for 18 h significantly reduced the ability of MAC-T cells to adhere to the cell culture dishes, prevented the growth of those cells that were attached to the dishes, and caused cell death. Supernatants from Hly⁺ and Hly^- following injury and during repair had no lethal effect on MAC-T cells. The effects of culture medium obtained after growth of the repaired Listeria cells on MAC-T cells were similar to those recorded for medium from the first 18 h growth for both strains, indicating that cells regain virulence potential once they have repaired and reinitiated growth. Culture filtrates obtained from L. monocytogenes Scott A showed results similar to those of Hly⁺, decreasing adherence and growth of MAC-T cells, while L. innocua culture filtrate had no adverse effect. The results of these experiments suggest that when injured, L. monocytogenes does not demonstrate adverse effects towards MAC-T cells. Once repair is completed and the listeria are growing, activity towards MAC-T cells is restored.