The discovery of a mysterious gene called Bphs transformed our understanding of why individuals respond differently to infections—revealing an unexpected connection between histamine sensitivity and autoimmune disorders.
Imagine two mice genetically identical in nearly every way, both exposed to the same bacterial toxin. One mouse experiences mild, fleeting symptoms while the other dies within hours from catastrophic shock. This dramatic difference in survival, traced to a single genetic variation, has unveiled fundamental new insights into how our bodies respond to disease—insights that extend far beyond the initial infection to illuminate the mysterious world of autoimmune disorders.
The Bphs gene, later identified as Hrh1, controls susceptibility to histamine sensitization after pertussis toxin exposure and influences autoimmune disease development.
For decades, scientists have known that Bordetella pertussis—the bacterium that causes whooping cough—produces a potent toxin that can trigger extreme sensitivity to histamine in some individuals but not others. The genetic basis for this difference remained elusive until researchers discovered Bphs, a gene that controls this life-or-death response. The subsequent identification of Bphs as the histamine H1 receptor gene (Hrh1) revealed an unexpected connection between a common neurotransmitter and our susceptibility to autoimmune conditions like multiple sclerosis. This is the story of how solving one genetic mystery opened new windows into understanding human health and disease.
Pertussis toxin (PTX) is a remarkable and complex bacterial protein that acts as a master manipulator of cellular communication. Structurally, it follows the A-B model common to many bacterial toxins: the 'A' subunit possesses enzymatic activity, while the 'B' subunit handles binding to target cells 2 .
Once inside the body, PTX disrupts normal cellular signaling by ADP-ribosylating the alpha subunit of trimeric Gi proteins 1 2 —essentially hijacking the internal communication systems of our cells.
The story begins in 1948, when researchers made a puzzling observation: mice that normally tolerated histamine injections would die from them if first exposed to Bordetella pertussis or its toxin 3 . This phenomenon was dubbed Bordetella pertussis-induced histamine sensitization (Bphs).
What made this observation particularly intriguing was that not all mice responded equally. Some strains exhibited extreme sensitivity to histamine after PTX exposure, dying rapidly from hypotensive and hypovolemic shock, while other strains remained completely resistant 1 3 .
What makes PTX particularly fascinating to geneticists is the tremendous variation in individual responses to these effects. This variation provided the crucial clue that genetic factors were at work in determining susceptibility.
Through careful genetic breeding experiments and analysis of different mouse strains, researchers initially mapped the Bphs locus to the central region of mouse chromosome 6 3 . This was an important first step, but the exact identity of the gene remained unknown.
The turning point came when scientists created congenic mouse strains—animals that were genetically identical except for a small chromosomal region containing the Bphs locus 1 2 .
These specially engineered mice allowed researchers to isolate the effects of Bphs from other genetic variables. One particular congenic line, C3H.SJL-BphssD, proved as susceptible to histamine sensitization as the susceptible SJL/J mice, confirming that this specific chromosomal region controlled the response 2 .
In a breakthrough discovery, researchers identified Bphs as the gene encoding the histamine H1 receptor (Hrh1) 1 3 4 . The H1 receptor is a G-protein-coupled receptor found throughout the body, including the brain, blood vessels, and immune cells. When activated by histamine, it triggers inflammatory responses and plays a key role in allergic reactions .
The critical difference between susceptible and resistant individuals came down to subtle variations in the receptor's structure. The Bphs-susceptible (BphsS) and resistant (BphsR) alleles differed at three key amino acid positions in the receptor's third intracellular loop—a region critical for signal transduction and receptor trafficking 3 4 .
| Allotype | Amino Acid at Position 263 | Amino Acid at Position 312/313 | Amino Acid at Position 330/331 | Response to PTX-induced Histamine Sensitization | Cell Surface Expression |
|---|---|---|---|---|---|
| HRH1S (Susceptible) | P (Proline) | V (Valine) | P (Proline) | Sensitive | Normal |
| HRH1R (Resistant) | L (Leucine) | M (Methionine) | S (Serine) | Resistant | Reduced |
These structural differences don't affect the receptor's ability to bind histamine or activate signaling pathways once at the cell surface. Instead, the resistant allotype shows altered intracellular trafficking, with more receptors retained in the endoplasmic reticulum and fewer reaching the cell surface 4 . This reduced surface expression likely explains the blunted response to histamine in resistant strains after PTX exposure.
To definitively establish the role of Bphs/Hrh1 in PTX responses, researchers designed a comprehensive series of experiments using both congenic mice and mice with a genetically disrupted Hrh1 gene (Hrh1tm1Wat) 1 2 .
The experimental approach was elegant in its simplicity—compare responses to various PTX effects in genetically matched mice differing only at the Bphs/Hrh1 locus.
The findings revealed a remarkable specificity in Bphs/Hrh1's functions. The gene influenced susceptibility to histamine hypersensitivity and enhanced antigen-specific delayed-type hypersensitivity responses (a form of cell-mediated immunity) but did not significantly affect other PTX responses 1 2 .
Perhaps most strikingly, the research demonstrated that hypersensitivity to serotonin—another vasoactive amine—was unaffected by Bphs/Hrh1, indicating that the genetic control of vasoactive amine sensitization is specific to particular mediators 1 .
These findings provided crucial insight: the variation in responsiveness to PTX reflects genetic control of distinct intermediate phenotypes rather than overall susceptibility to intoxication 1 .
| PTX-induced Response | Influenced by Bphs/Hrh1? | Key Experimental Evidence |
|---|---|---|
| Histamine hypersensitivity | Yes | Bphs congenic and Hrh1-knockout mice showed dramatically different responses |
| Enhanced delayed-type hypersensitivity | Yes | PTX's adjuvant effect was blunted in resistant strains |
| Serotonin hypersensitivity | No | Response patterns differed from histamine sensitivity |
| Lethal toxicity | No | Interstrain differences observed but not linked to Bphs/Hrh1 |
| Leukocytosis | No | All strains showed similar relative increases in white blood cells |
| Glucose regulation disruption | No | Blood glucose changes occurred independently of Bphs/Hrh1 status |
| Histamine-independent vascular permeability | No | Increased blood-tissue barrier permeability occurred in all strains |
Understanding how Bphs/Hrh1 functions required specialized experimental tools and approaches. The following table highlights some of the key resources that enabled this research:
| Reagent/Method | Function in Research | Specific Example/Application |
|---|---|---|
| Congenic mouse strains | Isolate effect of specific genetic locus on uniform background | C3H.SJL-BphssD mice (SJL Bphs allele on C3H/HeJ background) 2 |
| Gene-targeted knockout mice | Determine function of specific gene by examining its absence | B6.129P-Hrh1tm1Wat mice (Hrh1 disruption) 2 4 |
| PTX sensitization protocol | Standardize induction of histamine hypersensitivity | IV PTX followed by histamine challenge after 3-5 days 2 |
| Vascular permeability assay | Quantify changes in blood-tissue barrier function | 125I-labeled bovine serum albumin as radiotracer 2 |
| Delayed-type hypersensitivity measurement | Assess cell-mediated immune responses | Ear swelling after antigen challenge in sensitized mice 2 |
| Flow cytometry with HA-tagged receptors | Analyze receptor expression and trafficking | pEGZ-HA-H1R vectors for transfection studies 4 |
These specialized tools enabled researchers to move from observing phenotypic differences to understanding their molecular mechanisms—from noting that some mice died from histamine challenge after PTX exposure to explaining exactly why at the cellular level.
The story of Bphs/Hrh1 took an unexpected turn when researchers discovered its connection to experimental autoimmune encephalomyelitis (EAE), the primary animal model for multiple sclerosis 4 .
Mice with the susceptible Hrh1 allele developed more severe EAE, while those with the resistant allele or lacking the receptor entirely showed reduced disease severity 4 .
Even more remarkably, when researchers selectively reexpressed the susceptible Hrh1 allele only in T cells of knockout mice, they could restore full EAE susceptibility 4 . This demonstrated that Hrh1's effect on autoimmune disease operated specifically through its expression in T cells—a surprising finding for a receptor best known for its role in allergic responses.
The genetic story continues to evolve. In 2023, researchers made another surprising discovery: several wild-derived inbred mouse strains carrying the "resistant" Hrh1 allele nevertheless showed histamine sensitization 3 .
This paradox pointed to the existence of additional genetic factors that could modify the Bphs response.
Through careful genetic mapping, the team identified a modifier locus on mouse chromosome 6 they named Bphse (enhancer of Bordetella pertussis-induced histamine sensitization) 3 . This discovery highlights the complexity of genetic regulation and how multiple genes can interact to shape physiological responses.
Candidate genes within this modifier locus include genes involved in diverse cellular processes from autophagy to neuronal signaling. Identifying which of these genes modifies Hrh1 function represents the next chapter in this ongoing scientific story.
The journey to understand Bphs/Hrh1 illustrates how studying seemingly narrow biological questions—like why some mice die from histamine challenge after pertussis toxin exposure—can illuminate fundamental physiological principles with broad implications for human health.
The Bphs/Hrh1 story exemplifies how curiosity-driven research into a puzzling observation can evolve into insights with far-reaching implications—connecting whooping cough to multiple sclerosis, and mouse genetics to human medicine.
As research continues into modifier genes like Bphse and the precise mechanisms by which Hrh1 influences immune function, this story continues to unfold, reminding us that in biology, seemingly simple questions often lead to unexpectedly profound answers.