Man Flu Who?: Sex differences in immune response

Have you ever heard the term “Man Flu”? The Cambridge Dictionary defines it as “an illness such as a cold that is not serious, but that the person who has it treats as more serious, usually when this person is a man.” While this term is generally used in a humorous sense, there are actually sex-based differences in response to infectious disease! This is especially relevant during the COVID-19 pandemic, as men are at a higher risk for hospitalization, mechanical ventilation, and death.

What are the differences in immune systems between the sexes?

While everyone has the same immune cells in their body, males and females have different immune responses to stimuli. Did you know that 80% of autoimmune disease occurs in females? Females generally have stronger immune responses that allow them to fight pathogens (infectious microorganisms) quicker. This happens for a variety of reasons related to innate immunity, adaptive immunity, sex hormones, and chromosomes.

Innate immunity?

Innate immunity is known as the “first responder” to an infection in the body. It is fast acting, and responds to conserved features of pathogens. Conserved features are sequences of DNA, RNA, or proteins that are identical/similar across different types of pathogens. This means that while the innate immune responses have the advantage of speed, they do not have antigen specificity (antigens are unique molecular structures present on the surface of pathogens). The innate immune system is made up of physical (e.g. skin!) and chemical (e.g. blood clotting factors) barriers to keep out infectious agents, and it recruits immune cells to the site of infection via production of cytokines (molecular messengers between cells). It does not result in immunological memory, meaning that if the host is reinfected with the same pathogen again, the innate immune response intensity will still be the same as it was the first time.

So how do innate immune responses differ between males and females? To answer this question, let’s take a look at some important innate immune cells: macrophages, neutrophils, and dendritic cells. Macrophages (from Greek: makrós = large, phagein = to eat) are a type of white blood cell that engulfs and digests pathogens in a process called phagocytosis. Neutrophils are another type of phagocytic white blood cell (fun fact: they are the very first cells to be recruited from the bloodstream to infection sites!). In females, macrophages and neutrophils have demonstrated higher phagocytic activity than in males. One important difference between macrophages and neutrophils is that macrophages have antigen presenting capacity, whereas neutrophils do not. Antigen presenting cells (including macrophages, dendritic cells, and B cells) are immune cells that break down antigens and display the fragments to inform the adaptive immune system. Antigen presenting cells (APCs) from females are more efficient than APCs from males.

Macrophages and dendritic cells express an important class of proteins called Toll-like receptors (TLRs). TLRs recognize the conserved structures present on pathogens. The binding of a ligand to its TLR initiates the innate immune response. Humans have 10 TLRs; TLR3, 7, and 8 recognize viral RNA (their natural ligand). Once a TLR7 ligand has bound to its receptor on a plasmacytoid dendritic cell, interferon-α (IFN-α) gets produced. Plasmacytoid dendritic cells (pDC) are a type of dendritic cell that can produce high levels of interferon-α (IFN-α). Interferons are a family of cytokines with important antiviral effects. When IFN-α is produced by a pDC, it stimulates macrophages and natural killer cells to mount an antiviral response. Natural killer cells (NKs) are a cytotoxic type of white blood cell, meaning that they kill virus-infected host cells by producing small proteins that perforate the target cell and induce apoptosis. It has been shown that pDC derived from females produce significantly more IFN-α after TLR-7 ligand binding than in pDC derived from males. This means that, when infected with the same viral RNA, there will be higher stimulation of antiviral response in a female compared to a male.

Adaptive immunity?

An important role of the innate immune system is to activate the adaptive immune system (see above discussion on antigen presenting cells). In comparison to innate immunity, adaptive immunity responds slower, but it persists longer and generates immunological memory. Importantly, it has specificity to the pathogen causing the infection. If you were to think about innate immunity as the first responders to a crime scene (infection), then adaptive immunity would be like the detectives. They show up to the crime scene later and they have a description of the perpetrator (specificity for a pathogen). Then, they stick around and take notes for their case file (immunological memory).

Lymphocytes are a type of white blood cell that carry out adaptive immune responses. Lymphocytes come in two classes: B cells and T cells. B cells are responsible for antibody responses (also known as humoral immunity), so they secrete antibodies when activated. Antibodies are Y-shaped proteins called immunoglobulins that are used by the immune system to neutralize pathogens. When the antibody binds to its specific antigen on the pathogen, this inactivates the pathogen by preventing it from binding to receptors on host cells. On the other hand, T cells are responsible for cell-mediated immune response. This means that they act directly on the foreign antigen present on the host cell surface (via APCs). One major subtype is the CD8+ “killer” T cells which are cytotoxic, meaning they can kill virus-infected cells, thus preventing viral replication. Another type, CD4+ “helper” T cells produce cytokines that activate B cells, cytotoxic T cells, and many other effector cells.

In terms of sex differences, females usually have more vigorous humoral and cell-mediated immune responses to vaccines, antigens, and infections than males. For humoral immunity, this means that females have higher basal levels of immunoglobulin and higher antibody responses to vaccines. Females typically experience more adverse side effects after vaccination due to the reactivity of their immune system, but this makes them more resistant to infectious diseases! On the cell-mediated side, there is on average a higher frequency of CD4+ T cells circulating in females. So overall, there is enhanced cytokine production in response to infection in females, allowing them to better reduce and clear pathogens.

Sex Hormones & Chromosomes

The mechanisms underlying sexual dimorphism in immune response are related to sex hormones and chromosomes. Sex hormones are steroid hormones such as estrogen, progesterone, and testosterone that are present in different ratios between males and females. Endocrine-immune interactions occur when hormones bind to specific receptors to alter immune cell performance. For example, estrogen has an effect on the differentiation and activation of dendritic cells. When immature DCs are exposed to estrogen, they have enhanced cytokine secretion and increased T lymphocyte stimulatory capacity. When it comes to B lymphocytes, estrogen enhances IgG and IgM (two important immunoglobulins) production in both males and females. Testosterone inhibits production of these immunoglobulins both directly as well as indirectly by lowering the production of IL-6 (a cytokine) by monocytes (phagocytic white blood cells). Males usually produce more testosterone than females, thus they may have a more suppressed humoral immune response.

Sex differences in immune response can also be explained by genes present on chromosomes in XX females and XY males. The X chromosome is 5% of the human genome and has upward of 1200 genes, while the Y chromosome has at least 50-60 genes. A large number of the genes on the X chromosome are immune-related genes, meaning they encode proteins involved in immune functions. Included in the important immune-related genes are the Toll-like receptors TLR8 and TLR7. Since females have a higher expression of these TLR, their pDCs produce more IFN-α after stimulation when compared to males (as mentioned above in the innate immunity section). When it comes to immune system disease resulting from gene mutation, XX females have some protection in terms of X chromosome inactivation. This is when one copy of the X chromosome is packaged into heterochromatin and silenced, so that there is not double the number of X chromosome gene products. XY males only have one copy of the X chromosome, thus they are more vulnerable to immune dysregulation when there is a mutation in the X chromosome.

How is this relevant to COVID-19?

Large-scale global statistical analysis of COVID-19 infection data showed that while males and females were equally likely to get infected with the virus, the males were more likely to develop severe disease. Males had higher rates of intensive therapy unit (ITU) admission and a significantly increased risk of mortality. These findings may be explained by the sex differences in the immune systems and response. Some data shows that in moderate COVID-19 infection, male patients had higher levels of innate immune cytokines and robust induction of non-classical (anti-inflammatory) monocytes, while female patients had more robust T-cell activation. A poor T cell response was associated with a worse disease outcome in male patients but not female patients. On the other hand, higher levels of innate immune cytokines were associated with worse disease outcomes in females but not in males. This data is important because it highlights the benefits of taking a sex-dependent approach when considering prognosis, treatments, and vaccines.