Emma Sulkowicz, a senior visual arts student at Columbia University, carries a mattress on campus Sept. 5, 2014, in New York City. (Andrew Burton/Getty Images)

Last fall, Columbia University senior Emma Sulkowicz began carrying a mattress around campus — a protest, she said, of how the school handled her sexual assault complaint against a fellow student. She didn’t name her alleged assailant, but as Sulkowicz’s story and the image of her mattress went viral, his identity soon became obvious. By the end of the term, Paul Nungesser had been denounced on fliers and at rallies and former friends crossed the street to avoid talking to him.

Now Nungesser is suing his school, its board of trustees, its president and one of its professors, saying that Columbia failed to protect him from a “harassment campaign” by Sulkowicz even after a school disciplinary panel cleared him of responsibility in the case.

“Columbia University’s effective sponsorship of the gender-based harassment and defamation of Paul resulted in an intimidating, hostile, demeaning . . . learning and living environment,” reads the federal discrimination lawsuit filed Thursday, according to the Associated Press.

[The accuser and the accused have been fighting it out in public]

The lawsuit alleges that Nungesser’s rights were violated by Columbia and its officials for supporting Sulkowicz, and by professor Jon Kessler for approving the mattress-carrying piece as her senior thesis. Nungesser is a senior and German national, and his reputation and job prospects in the United States “are suffering immensely” because of the project, according to the lawsuit.

It’s true that Sulkowicz’s performance piece, dubbed “Carry that Weight,” has become something of a national phenomenon in the months since she launched the project. Images of Sulkowicz toting a 50-pound mattress into classrooms and up and down stairs went viral last September. Later in the fall, anti-sexual-assault groups at more than 130 schools organized a “Day of Action” inspired by the project — with students at each school carrying their own mattresses around campus. She has been profiled in the New York Times, New York Magazine, Time and elsewhere, and in January she attended the State of the Union as a guest of Sen. Kirsten Gillibrand (D-N.Y.). Nearly everyone knows Sulkowicz’s mattress, if not her name, and many see her as a victim of everything that’s wrong with college campus culture when it comes to rape.

Which would make Nungesser the villain — a characterization he vehemently denies in this lawsuit and elsewhere. He said that Sulkowicz’s project is designed to bully him, and that Columbia and its faculty have effectively endorsed that bullying by giving her course credit for it.

[It’s hard to ignore a woman toting a mattress everywhere she goes, which is why Emma Sulkowicz is still doing it]

The lawsuit added that a Columbia-owned Web site portrayed Sulkowicz’s version of the story — that Nungesser, a former friend, sexually assaulted her in 2012 — as fact, according to the Associated Press. It said that the university allowed Sulkowicz to carry her mattress into classes, the library and on campus-provided transportation, and that Sulkowicz’s pledge to carry her mattress at graduation may prevent Nungesser’s parents from participating in the ceremony.

In addition, Nungesser “has been subjected to severe, pervasive . . . and threatening behavior by other Columbia students, believing that Paul is a ‘serial rapist,’ whenever Paul has appeared at university activities,” the lawsuit said. (Two other Columbia students filed sexual assault complaints against Nungesser, though he was not found responsible in both of those cases.)

The lawsuit, which was filed in Federal District Court in Manhattan, seeks damages in an amount to be determined at trial for the harm it says was done to Nungesser’s reputation, his school opportunities and his job prospects.

It comes just days after a judge dismissed another lawsuit against Columbia by an unnamed male student. In that suit, the student alleged that the school violated his Title IX rights — treating him too harshly in a sexual assault hearing against him to counter perceptions that the school wasn’t doing enough to tackle campus rape.

Sulkowicz is not named as a defendant in the suit, which focuses instead on how Columbia has responded to her project.

“By refusing to protect Paul Nungesser,” the lawsuit reads, “Columbia University first became a silent bystander and then turned into an active supporter of a fellow student’s harassment campaign by institutionalizing it and heralding it.”

Roger Hornsby, a Columbia spokesman, told the AP that the school had no comment. In the past, Columbia President Lee Bollinger has also declined to comment on the issue, though he told the New York Times in December, “The law and principles of academic freedom allow students to express themselves on issues of public debate; at the same time, our legal and ethical responsibility is to be fair and impartial in protecting the rights and accommodating the concerns of all students in these matters.”

Sulkowicz found Nungesser’s suit “ridiculous,” she wrote in an e-mail to the AP.

“I think it’s ridiculous that Paul would sue not only the school but one of my past professors for allowing me to make an art piece,” she wrote. “It’s ridiculous that he would read it as a ‘bullying strategy,’ especially given his continued public attempts to smear my reputation, when really it’s just an artistic expression of the personal trauma I’ve experienced at Columbia. If artists are not allowed to make art that reflect on our experiences, then how are we to heal?”