New frontiers in suicide vulnerability: immune system and sex hormones

Suicide is one of the leading causes of death worldwide and men have a higher risk of attempting and completing suicide than women. Accumulating evidence leads to a possible key role of the immune system and sex hormones in psychiatric conditions associated with suicide vulnerability (e.g., major depressive disorder). Moreover, the literature highlights a dysregulation of the immune system and altered sex hormone levels in suicidal patients. Sex hormones and the immune system may have a role in suicide risk and sex differences in suicide vulnerability. This brief review emphasises a research area focused on a possible interplay between the immune system and sex hormones that may help develop a better understanding of suicide vulnerability in the perspective of sex-specific therapeutic approaches.

Suicide is one of the leading causes of death worldwide and men have a higher risk of attempting and completing suicide than women. Accumulating evidence leads to a possible key role of the immune system and sex hormones in psychiatric conditions associated with suicide vulnerability (e.g., major depressive disorder). Moreover, the literature highlights a dysregulation of the immune system and altered sex hormone levels in suicidal patients. Sex hormones and the immune system may have a role in suicide risk and sex differences in suicide vulnerability. This brief review emphasises a research area focused on a possible interplay between the immune system and sex hormones that may help develop a better understanding of suicide vulnerability in the perspective of sex-specific therapeutic approaches.

Suicide vulnerability
Historically, suicide and suicide attempts were criminalised with consequent actions against the attempters (if not completed suicide), the corpse, or on the family of the departed. Nowadays, however, many countries have decriminalised suicide, possibly leading to a subsequent decrease in suicide rates (Mishara and Weisstub 2016). The World Health Organisation (WHO) counts almost 800,000 suicide deaths every year (1.4% of worldwide deaths) (Roth et al., 2018;World Health Organisation 2014). The phenomenon of suicide is multi-faceted with different grades of severity (completed suicide, suicide attempts, suicidal ideation), and can be analysed from different perspectives (e.g., cultural, psychological, and biological). Specific domains may increase the risk of suicide in childhood and adolescence: psychological factors (e.g., psychiatric disorders, previous suicide attempts, substance use), stressful life events, personality traits (e.g., neuroticism), social and educational disadvantage, family history of suicidal behaviour, family-related factors, childhood trauma, and strong belief in healing practices (Beautrais 2000;Randell et al., 2006;Vang 2013;Carballo et al., 2020). Multiple spheres are associated with suicide in adulthood: mental health, alcohol/substance abuse, recent crisis or stress, financial issues, relationship problems, or medical conditions (Logan et al., 2011). Most deaths caused by completed suicide are associated with psychiatric conditions (e.g., bipolar disorder (BD), post-traumatic stress disorder (PTSD), and schizophrenia), and 50%-66% of completed suicides are associated with affective disorders, specifically with major depressive disorder (MDD) (Arsenault-Lapierre et al., 2004;Cavanagh et al., 2003;Skogman et al., 2004). Even though the segment of the population that shows increased suicide risk is between 35 and 44 years of age (Cash and Bridge 2009); young people (<25 years of age) with MDD show a higher suicide risk than adults (Blair-West et al., 1999).
Despite suicidal ideation and behaviour being associated with the female sex (Cantor, 2000), men show higher annual suicide rates and a higher frequency of attempted suicides than women (Barrig on and Cegla-Schvartzman 2020; Oquendo et al., 2001;Skogman et al., 2004). In a National Register Based analysis (Nordic countries), male sex, previous suicide attempts, the severity of depression, and substance abuse were identified as risk factors (Isomets€ a 2020). Nevertheless, Skogman and colleagues highlighted sex-specific risk factors: male-specific risk factors are suicide attempts and lethal methods; female-specific risk factors are and older age and suicidal intent (Skogman et al., 2004). Men are more likely to choose violent methods and are therefore more likely to complete suicide due to the more effective methods (Stenbacka and Jokinen, 2015). On the contrary, women are more likely to choose non-violent methods (e.g., carbon monoxide poisoning) (Denning et al., 2000). The increased suicide risk in male patients may be aggravated by sex differences in the duration of the suicidal process. The process from the onset of ideation to the suicide act itself is shorter (i.e., quicker) in men than women (Neeleman et al., 2004;Van Heeringen 2001), giving less chance of timely intervention. Moreover, depressed male patients tend to show higher irritability, hyperactive behaviour, a tendency to overreact, more frequent anger attacks, and lower impulse control than depressed female patients (Winkler et al., 2005). These traits may contribute to increased difficulty in reaching an early and prompt diagnosis of depression and to subsequent increased risk of completing suicide.
The high rates of suicide worldwide, the complexity of this phenomenon, and the correlation with mental health (in particular with depression (Lombardo 2019a)) led to research investigating the underlying biological mechanisms to better understand suicide vulnerability and then, to prevent suicide.

The role of the immune system in suicide risk
The role of the immune system is detectable across various psychiatric conditions associated with suicide vulnerability (e.g., schizophrenia and PTSD) (Balhara and Verma 2012;Gradus et al., 2010;Müller et al., 2015). In particular, the immune system is one of the main biological mechanisms currently under investigation in affective disorders. Experimental activation of the immune system can induce transient depressive symptoms ) and a recent large study in the UK Biobank found that both male and female depressed patients have increased inflammatory levels (Pitharouli et al., 2021). In fact, depressed patients show increased pro-inflammatory biomarker levels (e.g., C-reactive protein (CRP) and tumour necrosis factor (TNF)-α) (Osimo et al., 2020).
Treatment-resistant depressed (TRD) patients show even higher CRP levels (Chamberlain et al., 2019) than responders, with increased expression of immune genes (Cattaneo et al., 2020) and high rates of attempted/completed suicides (Bergfeld et al., 2018). Hence, the investigation into the role of the immune system in depression is a promising field for new target treatments and personalised medicine, with a view to reducing suicide rates. Moreover, baseline biomarker levels may affect the treatment response. For instance, research has recently begun to demonstrate that only depressed participants with higher cortisol levels at baseline assessment may benefit from cortisol synthesis inhibitors (Lombardo et al., 2019b). Furthermore, TRD patients with higher baseline inflammatory levels (high sensitivity (hs)-CRP ! 3 mg/L) may benefit from add-on treatment with minocycline, an antibiotic with anti-inflammatory and anti-depressant properties (Rosenblat and McIntyre, 2018;Nettis et al., 2021).
The immune system may also play a key role in suicide vulnerability (Brundin et al., 2017) (see Table 1). Suicidal ideation/suicide attempts are associated with increased levels of inflammatory biomarkers and stress, independently of psychiatric conditions (Enache et al., 2019) and severity of depressive symptoms (O'Donovan et al., 2013). Specifically, hs-CRP, Presumptive Stressful Life Events Scale, and the Daily Hassles and Uplifts Scale-revised independently predict suicidal risk (R2 ¼ 0.765) (Priya et al., 2016). Janelidze et al. (2011) detected higher levels of interleukin (IL)-6 and TNF-α, and lower levels of IL-2 in suicide attempters with depression than depressed patients without suicidal ideation and controls (Janelidze et al., 2011). Moreover, individuals who attempted suicide show lower GR-α mRNA and higher CRP and TNF-α mRNA than those individuals with suicidal ideation and no history of suicide attempts (and controls) (Melhem et al., 2017). In a post-mortem study focused on teenage suicide victims (age range 12-20 years old), they showed higher IL-1β, IL-6, and TNF-α mRNA levels than controls in Brodmann area 10 (Pandey et al., 2012). However, the understanding of suicide risk may be deepened by investigating those patients with low-moderate peripheral inflammation. In fact, the risk of completed suicide is quadrupled in patients with CRP >3 mg/L compared with patients with CRP <1 mg/L (Batty et al., 2016).
Interestingly, the literature highlights sex differences in the biochemical mechanisms associated with this phenomenon. Previous studies investigating inflammatory biomarkers in suicide risk have reported inconsistent findings in analysing the activation of the immune system in male and female patients.
Women but not men may show an association between systemic inflammation and increased suicide risk. For instance, females with higher white blood cell counts (!5.9x109 cells/L) have a 30% increased risk of death by suicide (Batty et al., 2018). Moreover, depressed female patients show a positive but low correlation between suicidal thoughts and TNF-α (r¼0.33 p¼0.03) (Birur et al., 2017). Additionally, K€ ohler-Forsberg and colleagues detected a positive association between Table 1 Summary of the main findings of the discussed studies investigating immune system and suicide vulnerability.   Interestingly, there may be a need to further investigate sex-specific biological and inflammatory profiles in suicidal patients. A recent study by Cabrera-Mendoza and colleagues detected sex-specific gene expression in suicidal patients: cell proliferation and immune response in women, and DNA binding and ribonucleic proteins in men (Cabrera-Mendoza et al., 2020). Tonelli et al. (2008) identified sex-specific markers in a post-mortem study investigating the mRNA cytokine expression in the prefrontal cortex. The authors detected increased IL-13 mRNA expression in male suicide victims and increased IL-4 and IL-3 mRNA expression in female suicide victims (in comparison with controls) (Tonelli et al., 2008). Moreover, in a cohort of female patients with mood and anxiety disorders, IL-8 was inversely associated with increased suicide risk in the ridge regression model (β¼À0.03AE0.02, p<0.05) (Keaton et al., 2019).
Furthermore, as mentioned earlier, males have a higher tendency to choose violent suicide methods compared with females. Lindqvist and colleagues detected higher levels of CSF IL-6 in violent-method suicide attempters than non-violent method suicide attempters (Lindqvist et al., 2009). Moreover, impulsivity and aggression are linked to completed suicide in males (Mann et al., 2005). Interestingly, increased levels of IL-6 are associated not only with violent suicide attempt methods but also with impulsivity (r¼0.39 p< 0.01) (Isung et al., 2014). Additionally, Noorozi and colleagues (2018) detected higher frequencies of A T A haplotype (rs4073, rs2227306, and rs1126647 respectively) of IL-8 polymorphisms in suicidal individuals who endeavoured hard methods than suicidal individuals who endeavoured soft methods (Noroozi et al., 2018).
This evidence highlights the need to detect sex-specific markers associated with suicide risk.
The aforementioned altered levels of and sex differences in inflammatory biomarkers in suicidal patients may be linked to the immunomodulator proprieties of gonadal hormones and a possible role of sex hormones in suicide vulnerability.

The role of sex hormones in suicide risk
Sex hormones and their fluctuations affect the neurobiology of affective disorders (Rubinow and Schmidt 2019), the sex differences in susceptibility to stress, and the increase in inflammatory levels (Slavich and Sacher 2019). Moreover, the literature highlights altered levels of gonadal hormones in both male and female suicidal patients (see Table 2). Therefore, the investigation of sex hormones may be a key factor in understanding sex differences and the inflammatory profile in suicidal patients.
In women, suicide attempts are linked to decreased levels of estrogen (Sublette 2020), and increased levels of testosterone (p¼0.017) (Zhang et al., 2015). This is detectable also in bipolar depression. BD female patients (depressive episode or mixed episode) show a positive association between testosterone levels and the number of suicide attempts (r¼0.408 p<0.01) (Sher et al., 2014). Moreover, the menstrual cycle may be a significant component in suicide risk (Leenaars et al., 2009). For instance, Behera et al. (2019) reported an increased risk of completed suicide among women during their menses and luteal phase (Behera et al., 2019). Additionally, patients with a number equal to or greater than 3 suicide attempts have higher levels of progesterone and a higher percentage of suicide attempts in the luteal phase than the follicular phase (Mousavi et al., 2014). On the contrary, Baca-Garcia and colleagues observed that female life phases characterised by low levels of progesterone (and estradiol), such as menopause, were associated with female attempted suicides (Baca-Garcia et al., 2010). In our recent systematic review focused on the interplay of sex hormones and the immune system in affective disorders, we suggest a protective role of exogenous female sex hormones in sub-groups characterised by estrogen deprivation .
In men, completed suicide rates and suicide attempts are linked to increased androgen levels (Sublette 2020;Zhang et al., 2015), and high androgen levels are associated with aggressive behaviour (Pompili et al., 2013). Interestingly, aggressiveness (and impulsivity) shows a positive and moderate association with testosterone/cortisol ratio (cerebrospinal fluid) in male suicide attempters (r¼0.67 p¼0.02) (Stefansson et al., 2016). In a cohort of BD patients (mixed group of bipolar depression I and bipolar depression II), male patients showed positive but low correlations between testosterone and manic episodes (r¼0.32 p¼0.02) and between testosterone and the number of suicide attempts (r¼0.35 p<0.01) (Sher et al., 2012). On the contrary, a mixed diagnosis male suicide attempter group (adjustment disorder, personality disorder, MDD, schizophrenia) showed lower testosterone levels than controls; with a significant difference in schizophrenic patients and a statistical trend in MDD patients (both diagnosis groups revealed lower testosterone levels than controls). Nevertheless, violent-method suicide attempters showed lower testosterone levels than non-violent method suicide attempters (Tripodianakis et al., 2007).
Several studies detect an immune-modulating role of gonadal hormones, as we extensively discuss in our recent aforementioned review . Specifically, estrogen can have both anti-inflammatory and pro-inflammatory properties, depending on various aspects, such as the target organ or concentration (high vs. low) (Straub 2007). Testosterone is known to have mainly anti-inflammatory properties (e.g., by inhibiting TNF-α, IL-6, IL-1 expression). For instance, low testosterone levels are associated with pro-inflammatory biomarker expression (Bianchi 2019;Bobjer et al., 2013). Specifically, testosterone replacement administration in men with androgen deficiency reduced pro-inflammatory cytokines (i.e., TNF-α and IL-1β), and increased anti-inflammatory cytokines (i.e., IL-10) (and cholesterol) (Malkin et al., 2004). However, there is still a need for research measuring androgen in females and estrogen measure analyses in males, and for the selection of patients with moderate-high inflammatory levels at the baseline for a better understanding of suicide vulnerability. In fact, the literature also highlights a pro-inflammatory effect of testosterone in women and a pro-inflammatory effect of estrogen in men. Testosterone (along with androstenedione and dehydroepiandrosterone sulphate) shows a positive correlation with white blood cell count and it is a predictor of leukocyte count in women with polycystic ovary syndrome (PCOS) (Rudnicka et al., 2020). In men with rheumatoid arthritis, estradiol shows a positive and strong correlation with inflammation (Tengstrand et al., 2003). Specifically, the balance between estradiol and testosterone may be relevant in modulating the immune system. In fact, van Koeverden and colleagues detected a negative correlation between low testosterone/estradiol ratio and systemic inflammation in men with atherosclerotic disease (Van Koeverden et al., 2019).

Challenges for future research
There is a lack of studies in this area which makes it difficult to reach a clear understanding of the interplay between sex hormones and the immune system in suicidal patients. Moreover, the reported information is association data, and the strength of these associations is low-moderate. Thus, it is necessary to further evaluate the strength of these associations, for example by following the Bradford Hill criteria (Hill 1965;Fedak et al., 2015), using systematic review/meta-analysis formats and a systematic approach. Most studies did not stratify, nor select, the patients  based on inflammatory biomarker levels. In fact, selecting suicidal patients who attempted suicide or who show suicidal intent, may help in clarifying the role of altered inflammatory biomarker and sex hormone levels (and their interplay) in this population. Furthermore, the inconsistency across the literature investigating inflammatory biomarker levels in suicide vulnerability needs further investigation. Moreover, the plethora of factors involved in this phenomenon requires an investigation of, and comparison between, the role of biological and non-biological (sociocultural, religious, and behavioural) factors, as well as sex differences, involved in suicide vulnerability.

Conclusion
The literature highlights that altered sex hormone levels correlate with increased suicide risk (e.g., decreased estrogen levels in female patients correlate with suicide attempts and increased androgen levels may play a role in higher rates of completed suicides in men). Furthermore, several studies reported altered immune system activation in suicide attempters/victims, also with differences in inflammatory profiles between suicide methods (i.e., violent vs. non-violent). However, even though several studies described the immune modulator role of gonadal hormones, there are no studies in the literature to our knowledge that specifically investigate the interplay between the two factors of interest (i.e., sex hormones and immune system) in suicide risk and sex differences in suicide vulnerability.
Given the altered levels of inflammation and sex hormones in suicidal patients (and in psychiatric conditions associated with suicide risk), lack of consideration for the interaction between the immune system and gonadal hormones can limit the progress in knowledge of suicide vulnerability and can preclude new preventive tailored therapeutic strategies (see Fig. 1).

Declaration of competing interest
The author declares that there is no conflict of interests. While completing her Master of Psychology at the University of Turin (Italy) "Body and Mind Sciences" course, with a particular interest in the dialogue between biological and psychological aspects in mental health, she worked as a research assistant at the King's College London to follow her passion in the interplay between mind and body. She believes that the study of biological dynamics in psychiatric illnesses will help in creating a sex-specific medicine and tailored treatments in order to offer the best rehabilitation pathway for those patients in need. Therefore, after the Master, Giulia decided to pursuit her passion in her PhD project investigating the (Barrigon and Cegla-Schvartzman, 2020) role of inflammation and sex hormones in depression and in sex differences in treatment response, with a particular focus on major depressive disorder and treatment resistant depression. Giulia is also one of the main academic leads of the Immunopsychiatry Meetings organised by the PIXIELab, which represent an opportunity for discussion and debate with the aim to create an international network of Immunopsychiatry.