CD8 is a type I transmembrane glycoprotein of the immunoglobulin family that plays an integral role in signal transduction, T cell differentiation, and activation. It exists on the cell surface primarily as either a disulfide-linked heterodimer (CD8αβ) or homodimer (CD8αα). The CD8α chain is essential for binding to MHC class I molecules, where it functions as a co-receptor alongside the T cell receptor (TCR) . CD8 is predominantly expressed on cytotoxic T lymphocytes (CTLs), certain subpopulations of αβ T cells and γδ T cells, and some NK cells .
The importance of CD8 in research stems from its critical role in the adaptive immune response. When activated through ligation of MHC-I/peptide complexes presented by antigen-presenting cells, CD8+ T cells trigger recruitment of lymphocyte-specific protein tyrosine kinase (Lck), leading to lymphokine production, motility, and CTL activation . These activated CTLs are crucial for clearing pathogens and tumor cells, making CD8 an essential marker for studying immune responses to infections and cancer .
FITC (Fluorescein Isothiocyanate) is one of the most widely used fluorochromes for antibody conjugation due to several advantageous properties. It provides relatively high absorptivity, excellent fluorescence quantum yield, and good water solubility . These characteristics make FITC-conjugated antibodies reliable tools for flow cytometry applications.
Different CD8 antibody clones recognize distinct epitopes of the CD8 molecule, which affects their applications and performance:
Clone 3B5: This monoclonal antibody targets CD8 and is commonly used for flow cytometry applications. It recognizes CD8α and is suitable for identifying CD8+ T lymphocytes in research settings .
Clone HIT8a: This clone specifically binds to CD8α and is noted for not cross-blocking with other clones like RPA-T8. This characteristic makes it valuable for co-staining experiments where multiple CD8 epitopes need to be detected simultaneously .
Clone MEM-31: This antibody recognizes a conformationally-dependent extracellular epitope of CD8. Importantly, it does not react with formaldehyde-fixed cells and is negative in Western blotting applications. This makes it specifically suitable for flow cytometry applications with fresh or appropriately preserved samples .
Clone EP72: This monoclonal antibody has been validated for flow cytometry and immunohistochemistry on frozen sections (IHC-Fr), particularly with chicken samples. Its specific binding properties make it suitable for these particular applications and species .
When selecting a clone, researchers should consider the specific application, sample preparation method, and epitope accessibility in their experimental conditions.
When using CD8-FITC antibodies for flow cytometry, researchers should follow these methodological steps:
Sample Preparation:
Staining Protocol:
Use pre-diluted antibodies at the recommended volume per test
Include appropriate isotype controls at the same concentration as your antibody of interest
For surface staining, incubate cells with CD8-FITC antibody for 20-30 minutes at 4°C in the dark
Wash cells 2-3 times with flow cytometry buffer to remove unbound antibody
If performing multi-color analysis, consider compensation controls to address spectral overlap
Instrument Setup and Analysis:
Configure your flow cytometer according to the fluorochrome spectra
Use proper compensation settings if multiple fluorochromes are employed
Analyze your data using appropriate gating strategies to identify CD8+ populations
Quality Control Considerations:
Include viability dyes to exclude dead cells
Use FMO (fluorescence minus one) controls when establishing complex panels
Consider the expression level of CD8 when interpreting results—CD8 is typically highly expressed on cytotoxic T cells but may vary in different cell subsets or activation states
Remember that antibody performance can vary between lots and manufacturers, so validation with appropriate controls is essential before conducting critical experiments.
Incorporating CD8-FITC antibodies into multi-parameter flow cytometry requires careful panel design that considers marker expression levels, fluorochrome brightness, and potential spectral overlap. A systematic approach involves:
Marker Prioritization Using the Tier System:
CD8 typically falls into the primary tier of phenotypic markers used for basic cell identification
Secondary tier markers might include activation and exhaustion markers
Tertiary tier markers, which are often your experimental variables with lower expression, should be paired with the brightest fluorochromes
Strategic Fluorochrome Selection:
Since CD8 is usually highly expressed, FITC is generally suitable despite its moderate brightness
Reserve brighter fluorochromes (PE, APC, PE-Cy7) for markers with lower expression levels
Consider the specific optical configuration of your flow cytometer to optimize detection sensitivity
Managing Spectral Overlap:
FITC emission overlaps primarily with PE, so plan your panel accordingly
When using FITC with other green-yellow fluorochromes, ensure proper compensation controls
Single-stained controls should be prepared for each fluorochrome in your panel
Panel Validation Strategy:
Test the full panel on control samples before proceeding to valuable research samples
Evaluate potential fluorescence spread and adjust the panel if certain markers show significant interference
Compare the performance of markers when stained individually versus in the full panel
For example, when studying CD8+ T cell exhaustion, a panel might include:
CD8-FITC (primary tier)
CD3-APC (primary tier)
PD-1-PE (secondary tier, activation/exhaustion)
CD137-PE-Cy7 (tertiary tier, activation marker for tumor-reactive T cells)
This approach ensures optimal detection of all markers while minimizing interference between fluorochromes.
Identifying and characterizing distinct CD8+ T cell subsets requires thoughtful experimental design and interpretation of CD8-FITC antibody staining in conjunction with other markers. Key considerations include:
Differential Expression Patterns:
CD8 exists as both αα homodimers and αβ heterodimers, with different distributions across T cell subsets
CD8αβ heterodimers are predominantly found on conventional αβ T cells, while some γδ T cells and NK cells express CD8αα homodimers
Consider using antibodies that can distinguish between these forms if relevant to your research question
Subset-Defining Marker Combinations:
Differentiation states: CD8+ T cells can be classified as early-differentiated (CD27+CD8+CD57−), intermediate-differentiated (CD27+CD8+CD57+), and terminally-differentiated (CD27−CD8+CD57+)
Functional phenotypes: Additional markers like FOXP3 can identify regulatory CD8+ T cells, with CD57−FOXP3+CD8+ and CD57+FOXP3+CD8+ T cells showing different prognostic associations
Activation/exhaustion states: Markers such as PD-1, CD39, CD160, TIM-3, TIGIT, and TOX help identify activated and exhausted-like CD8+ T cells
Functional Correlation:
Context-Specific Interpretations:
CD137+ expression identifies tumor-reactive CD8+ T cells with an activated and exhausted-like phenotype that show superior anti-cancer activity in adoptive transfer models
The same phenotypic markers may have different implications depending on the disease context (cancer, chronic infection, autoimmunity)
This multi-dimensional approach allows for more precise identification of functionally relevant CD8+ T cell populations beyond what a single CD8-FITC staining could provide.
Several artifacts and limitations may affect the interpretation of CD8-FITC antibody staining in flow cytometry. Addressing these requires methodological rigor:
Autofluorescence Management:
FITC emission overlaps with the autofluorescence spectrum of many cell types, particularly activated or aged cells
Methodological solution: Include unstained controls for each sample type and consider using spectral flow cytometry with autofluorescence extraction algorithms
For tissues with high autofluorescence, consider alternative fluorochromes with emission in different spectral regions
Epitope Masking and Accessibility Issues:
Some CD8 antibody clones (e.g., MEM-31) recognize conformationally-dependent epitopes that may be affected by fixation procedures
Methodological solution: Optimize fixation protocols or select clones known to work with your fixation method
For intracellular staining protocols, verify that the CD8 epitope remains accessible after permeabilization
Spectral Compensation Challenges:
FITC compensation can be particularly challenging when used alongside PE or other green-yellow fluorochromes
Methodological solution: Use single-stained controls for each fluorochrome on the same cell type as your experimental samples
Consider computational approaches like automated compensation algorithms for complex panels
Antibody Internalization During Processing:
CD8 receptors can be internalized upon activation or during certain processing steps
Methodological solution: Minimize processing time, maintain samples at 4°C, and consider kinetic studies to understand potential internalization effects
For activated cells, comparing CD8 surface expression over time can help interpret apparent changes in staining intensity
Clone-Specific Limitations:
By anticipating these potential issues and implementing appropriate controls and optimization steps, researchers can enhance the reliability of their CD8-FITC flow cytometry data.
CD8-FITC antibodies have become integral tools in cutting-edge tumor immunology research, with several advanced applications:
Identification of Tumor-Reactive CD8+ T Cell Populations:
CD137+ (4-1BB) expression on CD8+ T cells helps identify tumor-reactive populations with superior anti-cancer activity
Methodology: Multi-parameter flow cytometry combining CD8-FITC with markers like CD137, PD-1, CD39, and other exhaustion markers enables identification of these specialized cells
These CD137+CD8+ T cells display a highly proliferative, fully activated effector and exhausted-like phenotype with enhanced expression of PD-1, CD39, CD160, TIM-3, TIGIT, TOX, and CD57
Prognostic Assessment of CD8+ T Cell Phenotypes:
Different CD8+ T cell phenotypes correlate with disease outcomes in cancer patients
Terminally-differentiated CD8+ T cells (CD27−CD8+CD57+) associate with longer progression-free survival in some cancers
Conversely, CD57−FOXP3+CD8+ T cells correlate with shorter progression-free survival and represent an independent poor prognostic factor
Methodology: Flow cytometric or immunohistochemical analysis of tumor-infiltrating lymphocytes using CD8-FITC combined with differentiation and regulatory markers
Adoptive Cell Therapy Development:
CD8-FITC antibodies help isolate and characterize CD8+ T cell populations for adoptive transfer
CD137+CD8+ T cells have demonstrated superior anti-cancer activity in humanized mouse models
Methodology: Flow cytometry-assisted cell sorting using CD8-FITC along with activation markers enables isolation of specific subsets for expansion and therapeutic use
Mice receiving adoptively transferred CD137+CD8+ T cells showed reduced tumor growth and higher CD8+ T cell tumor infiltration compared to those receiving CD137−PD-1−CD8+ T cells or bulk CD8+ T cells
Humanized Mouse Model Development:
CD8-FITC antibodies facilitate tracking of human CD8+ T cell responses in humanized mouse models
These models provide tools for immunotherapy research by enabling the study of human T cell subsets like activated and exhausted-like effector CD8+ T cells
Methodology: Flow cytometric analysis of human CD8+ T cell differentiation into phenotypes like terminal exhausted (Tex-term) and tissue-resident (TRM) cells in tumor-bearing humanized mice
These advanced applications demonstrate how CD8-FITC antibodies contribute to our understanding of tumor immunology and the development of novel immunotherapeutic approaches.
Optimizing CD8-FITC antibody staining requires systematic troubleshooting of several variables:
For Weak Signal Issues:
Antibody Titration: Perform a titration series (e.g., 1:50, 1:100, 1:200, 1:400) to identify the optimal concentration that maximizes signal-to-noise ratio
Incubation Conditions: Extend incubation time to 45-60 minutes at 4°C in the dark to enhance binding while minimizing internalization
Buffer Optimization: Use high-quality flow cytometry buffer with freshly added protein (2% BSA or FBS) to prevent non-specific binding while maintaining epitope accessibility
Sample Handling: Minimize processing time, maintain sample viability, and avoid repeated freeze-thaw cycles of antibodies
Clone Selection: Test alternative CD8 antibody clones if epitope accessibility might be an issue with your current clone
For High Background Issues:
Fc Receptor Blocking: Pre-block samples with Fc receptor blocking reagents (10-15 minutes before antibody addition)
Washing Protocol: Implement more stringent washing (3-4 washes with larger volumes) to remove unbound antibody
Viability Dye: Include a viability dye to exclude dead cells, which often exhibit high autofluorescence and non-specific binding
Filtration: Filter samples through a 35-70μm mesh before acquisition to remove aggregates
Compensation Adjustment: Carefully review and adjust compensation settings, as FITC spectral overlap can contribute to apparent high background in other channels
Instrument-Related Optimization:
PMT Voltage: Adjust photomultiplier tube voltage for optimal detection of FITC signal
Threshold Settings: Optimize threshold settings to exclude debris while capturing all cells of interest
Regular Calibration: Ensure regular calibration of the flow cytometer using standardized beads
Sample-Specific Considerations:
Autofluorescence Reduction: For highly autofluorescent samples (like lung or skin cells), consider autofluorescence quenching reagents
Alternative Fluorochromes: In cases of persistent autofluorescence issues, consider switching from FITC to fluorochromes with different spectral properties
By systematically addressing these variables, researchers can significantly improve CD8-FITC antibody staining quality and reliability.
A comprehensive control strategy is essential for reliable interpretation of CD8-FITC antibody staining in multi-parameter flow cytometry:
Essential Control Types:
Isotype Controls: Include a FITC-conjugated isotype-matched control antibody at the same concentration as the CD8-FITC antibody to assess non-specific binding
Unstained Controls: Prepare samples with no antibodies to establish baseline autofluorescence
Single-Stained Controls: For each fluorochrome in your panel, prepare a single-stained sample for compensation setup
Fluorescence Minus One (FMO) Controls: Include controls where each sample contains all fluorochromes except one to accurately set gating boundaries, especially for markers with continuous expression patterns
Biological Controls:
Positive Control Samples: Include samples known to contain CD8+ cells (e.g., peripheral blood lymphocytes) to confirm antibody functionality
Negative Control Samples: Use cell types known not to express CD8 (e.g., CD4+ sorted T cells) to verify specificity
Blocking Controls: For verification of specificity, include samples pre-blocked with unconjugated CD8 antibody before adding CD8-FITC
Quality Control Measures:
Viability Discrimination: Include a viability dye to exclude dead cells, which often exhibit altered marker expression and increased non-specific binding
Doublet Exclusion: Implement FSC-H vs. FSC-A or similar gating strategies to exclude cell doublets that can confound results
Time Parameter Monitoring: Record time during acquisition to identify potential flow interruptions or instrument issues
Standardization Controls:
Antibody Capture Beads: Use anti-mouse Ig beads labeled with CD8-FITC for consistent instrument setup across experiments
Reference Standards: Include stabilized control cells or commercial control samples when available for longitudinal consistency
Internal Controls: When applicable, include spike-in control cells with known CD8 expression levels to normalize across batches
This comprehensive control strategy allows for accurate data interpretation and troubleshooting of potential issues with CD8-FITC antibody staining.
Fixation and permeabilization can significantly impact CD8-FITC antibody staining characteristics, requiring careful protocol selection:
Effects of Different Fixation Methods:
Paraformaldehyde (PFA)/Formaldehyde:
Alcohol-Based Fixatives:
Methanol or ethanol fixation can denature certain CD8 epitopes
May be unsuitable for certain CD8 antibody clones that recognize conformational epitopes
Often used for intracellular staining protocols, requiring verification of compatible CD8 antibody clones
Sequential Staining Strategies:
Surface-First Approach:
Stain with CD8-FITC before fixation/permeabilization to preserve epitope recognition
Verify FITC fluorescence stability through your specific fixation protocol
May require higher initial antibody concentration to account for potential signal loss
Post-Fixation Staining:
Some epitopes remain accessible after fixation but before permeabilization
Test different fixatives and concentrations to determine optimal conditions for your specific CD8-FITC antibody clone
Protocol-Specific Considerations:
Intracellular Cytokine Staining:
CD8 surface staining should generally precede fixation/permeabilization
Brief fixation (10-15 minutes with 2-4% PFA) typically maintains CD8-FITC signal
Commercial fixation/permeabilization kits often provide optimized protocols for maintaining surface marker detection
Transcription Factor Staining:
Methodological Solutions:
Antibody Cocktail Optimization:
For multi-step protocols, determine whether CD8-FITC performs better in pre- or post-fixation cocktails
Some protocols may benefit from restaining surface markers after fixation/permeabilization
Clone Selection Based on Protocol:
By understanding these interactions and testing protocols systematically, researchers can optimize CD8-FITC antibody performance in experiments requiring fixation and permeabilization.
Analyzing CD8+ T cell subpopulations with heterogeneous CD8 expression requires sophisticated approaches:
Gating Strategies for Variable Expression:
Biexponential Display: Use biexponential scaling rather than logarithmic scaling to better visualize the full range of CD8 expression
Contour Plots: Employ contour plots with percentage gating to identify populations that might be obscured in dot plots
Density-Based Clustering: Consider computational approaches like t-SNE or UMAP for unbiased identification of populations with subtle differences in CD8 expression
Resolving CD8dim Populations:
Additional Markers: Incorporate lineage-specific markers to confirm the identity of CD8dim populations
Back-Gating Analysis: After identifying cell populations based on other markers, back-gate onto CD8 expression to characterize CD8 levels in different functional subsets
Reference Populations: Use internal reference populations with stable CD8 expression to normalize and interpret CD8dim signals
Multi-Parameter Analysis Approaches:
Co-expression Patterns: Analyze CD8 expression in conjunction with markers of differentiation states (CD27, CD57) or activation (CD137)
Boolean Gating: Create combinatorial gates to identify complex phenotypes like CD27−CD8+CD57+ (terminally differentiated) versus CD27+CD8+CD57− (early differentiated) T cells
Hierarchical Gating: Implement a hierarchical gating strategy starting with lineage markers before examining CD8 expression levels
Quantitative Assessment Methods:
Mean Fluorescence Intensity (MFI): Report CD8 expression levels as MFI for different subpopulations to quantify expression differences
Molecules of Equivalent Soluble Fluorochrome (MESF): Convert fluorescence intensity to standardized MESF values for more precise quantification across experiments
Receptor Occupancy Calculation: For certain applications, calculate the percentage of occupied CD8 receptors using saturating concentrations of antibodies
Context-Specific Interpretation:
Activation-Induced Modulation: Account for CD8 downregulation upon T cell activation when interpreting reduced CD8 staining
Tissue-Specific Variations: Recognize that CD8 expression levels may vary between blood, lymphoid tissues, and tissue-resident populations
Species Differences: Consider species-specific patterns of CD8 expression when working with models beyond human samples
By implementing these best practices, researchers can accurately identify and characterize CD8+ T cell subpopulations despite variations in CD8 expression levels.
Technological advancements in flow cytometry are expanding the applications and enhancing the utility of CD8-FITC antibodies in several ways:
Spectral Flow Cytometry Advancements:
Full spectrum analysis allows better resolution of FITC signal from autofluorescence, improving the signal-to-noise ratio
Unmixing algorithms can separate FITC signal from spectrally adjacent fluorochromes more effectively
These advances enable more complex panels incorporating CD8-FITC alongside fluorochromes that would traditionally show significant spectral overlap
Single-Cell Multiomics Integration:
Flow cytometry index sorting with CD8-FITC can be paired with single-cell RNA sequencing to correlate protein expression with transcriptional profiles
CITE-seq and similar technologies allow simultaneous detection of CD8 protein expression and mRNA transcripts
These integrated approaches provide deeper insights into the functional heterogeneity of CD8+ T cell populations
High-Dimensional Data Analysis Tools:
Machine learning algorithms can identify novel CD8+ T cell subsets based on complex marker combinations that include CD8-FITC
Dimensionality reduction techniques like t-SNE, UMAP, and FlowSOM facilitate visualization and interpretation of high-parameter data
These computational approaches are particularly valuable for identifying functionally distinct CD8+ T cell populations like tumor-reactive CD137+CD8+ T cells
Imaging Flow Cytometry Applications:
Combines traditional flow cytometry with microscopy to provide morphological information alongside CD8-FITC fluorescence data
Enables assessment of CD8 receptor localization, clustering, or internalization in different functional states
Particularly valuable for studying immunological synapse formation in CD8+ T cells
Translational Research Applications:
More sensitive detection systems enable better identification of rare CD8+ T cell populations with clinical significance
Standardization efforts improve inter-laboratory reproducibility, facilitating multi-center clinical studies
These advances support the development of CD8+ T cell-based cellular therapies, such as those leveraging the superior anti-cancer activity of CD137+CD8+ T cells
As flow cytometry technology continues to evolve, researchers can expect improved sensitivity, greater panel complexity, and enhanced integration with other technologies when using CD8-FITC antibodies, ultimately advancing our understanding of CD8+ T cell biology in health and disease.
Emerging trends suggest several promising future directions for CD8-FITC antibody applications in immunotherapy research:
Precision Immunophenotyping for Personalized Immunotherapy:
High-dimensional phenotyping using CD8-FITC alongside markers of activation, exhaustion, and tumor reactivity (e.g., CD137)
Correlation of CD8+ T cell phenotypes with treatment outcomes to identify predictive biomarkers
Development of patient-specific immunotherapy strategies based on CD8+ T cell subset analysis
Advanced Adoptive Cell Therapy Applications:
Isolation and expansion of specific CD8+ T cell subsets with superior anti-cancer activity, such as CD137+CD8+ T cells
Real-time monitoring of adoptively transferred CD8+ T cells to assess persistence, function, and tumor infiltration
Engineering of CD8+ T cells with enhanced functionality based on insights from comprehensive phenotyping
Tissue-Resident Memory T Cell Research:
Combination Therapy Optimization:
Evaluation of how checkpoint inhibitors, cytokine therapies, and targeted drugs modulate different CD8+ T cell subsets
Identification of optimal combination strategies based on CD8+ T cell phenotype changes
Development of sequential therapy approaches guided by CD8+ T cell functional status
Single-Cell Multi-Omics Integration:
Correlation of CD8 protein expression with transcriptomic, epigenomic, and proteomic profiles at the single-cell level
Identification of molecular drivers of functionally distinct CD8+ T cell states
Development of computational models to predict CD8+ T cell functionality and therapeutic potential
Humanized Model Refinement:
CD8 (Cluster of Differentiation 8) is a transmembrane glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). It plays a crucial role in T cell signaling and aids in cytotoxic T cell-antigen interactions . CD8 is predominantly expressed on the surface of cytotoxic T cells, but it can also be found on natural killer cells, cortical thymocytes, and dendritic cells . The CD8 molecule is a marker for the cytotoxic T cell population and is involved in recognizing antigens presented by MHC class I molecules .
Mouse anti-human antibodies are monoclonal antibodies produced in mice that are specific to human antigens. These antibodies are widely used in research and clinical diagnostics due to their specificity and ability to bind to human proteins . However, one challenge with using mouse-derived antibodies in humans is the potential for the human immune system to recognize these antibodies as foreign, leading to the production of human anti-mouse antibodies (HAMA) . This response can reduce the effectiveness of the treatment and cause adverse reactions .
Fluorescein isothiocyanate (FITC) is a derivative of fluorescein used in various applications, including flow cytometry and fluorescence microscopy . FITC is reactive towards nucleophiles, such as amine and sulfhydryl groups on proteins, allowing it to be used as a fluorescent label . It has excitation and emission spectrum peak wavelengths of approximately 495 nm and 519 nm, respectively, giving it a green color . FITC is prone to photobleaching, which can be a limitation in some experiments .
The combination of CD8, Mouse Anti-Human, FITC refers to a monoclonal antibody specific to the human CD8 protein, produced in mice, and conjugated with FITC for fluorescence detection. This reagent is commonly used in immunofluorescence assays, flow cytometry, and other applications where the detection and quantification of CD8+ T cells are required. The FITC conjugation allows for the visualization of CD8+ cells under a fluorescence microscope or the quantification of these cells using flow cytometry .
In summary, CD8, Mouse Anti-Human, FITC is a valuable tool in immunological research and clinical diagnostics, enabling the study of cytotoxic T cells and their role in the immune response.